Notes

At head of title: State of Florida, Department of Natural Resources, Division of Resource Management, Florida Geological Survey.

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edited by Thomas M. Scott and Warren D. Allmon.

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The Florida Geological Survey, Division of Resource
Management, Department of Natural Resources, is publishing, as its
Special Publication 36, Plio-Pleistocene Stratigraphy and
Paleontology of Southern Florida. This publication is a series of
papers by geologists currently involved in investigating the
Pliocene and Pleistocene deposits of southern Florida. Knowledge
of these deposits allows geologists to better understand the last
five million years of Florida's geologic history.

It is a common comment when two or
more scientists in almost any field get together
that "it's surprising that no one has ever done X".
At first glance, this remark seems particularly
applicable to the Plio-Pleistocene geology of
southern Florida. While the abundant and
beautifully preserved fossils of this region have
attracted the attention of amateurs and pro-
fessionals alike for more than a century, relatively
little is really known with any degree of con-
fidence about either the stratigraphy or the
geological history of the area.

More familiarity with both the hands-on
geology and the history of research in southern
Florida, however, can also lead one to think that
this situation is really not so surprising after all,
and that, for at least two principal reasons, it is
rather understandable that we do not have as
good an understanding of the late Cenozoic
geology of this area as we would like.

The first reason is that the incredibly
abundant, diverse and well-preserved fossils of
the southern Florida Plio-Pleistocene, the most
conspicuous aspect of the section and the one
that has attracted most workers to it, has without
doubt distracted attention from the strata in which
they are contained. Because the fossils are so
outstanding, paleontologists have been among
the most active geologists in the area; they quite
logically erected stratigraphic schemes that
emphasized fossils. Non-paleontological
geologists have largely ignored the region. The
exception has been hydrogeologists, who, in the
absence of existing lithostratigraphic schemes,
have tended to erect their own hydrostratigraphic
frameworks, largely in isolation from biostra-
tigraphy.

The "operational stratigraphic units" of the
Plio-Pleistocene have thus tended to be either
essentially faunal or hydrostratigraphic units, with
only tenuous if any explicit linkages between
them. Because they were often given names,
formally or informally, these units inevitably came
to be designated as lithostratigraphic units (e.g.,
the "Bermont Formation").

The second reason for the state of the

stratigraphic art in southern Florida is a much
more straightforward, but much less-often
articulated one: the Plio-Plelstocene geology of
the southern half of the Florida peninsula is
complex. We now know enough about the
stratigraphy in this area to know that it is
characterized by great lateral and vertical
heterogeneity, evidently caused by rapid faces
changes and complex diagenesis. In addition, the
geologic data base from which one can interpret
the geology of the area is extremely limited.
There are precious few exposures, both man-
made and natural, and a very sparse collection of
subsurface samples from which data may be
obtained.

Why does all this matter? Given that the
Plio-Pleistocene geology of southern Florida is
poorly known, for whatever combination of
reasons, the same could be said for many other
regions of the world. Why is it worth the time and
the effort to unravel the geological history of this
particular spot?

The Plio-Pleistocene was a time of crucial
change in world climates, changes that affected
the physical and biological makeup of Florida as
well as most of the rest of the world. The global
warmth and equitability that characterized most of
the Cenozoic gave way to the cooling and insta-
bility that now characterize world climates. In the
western Atlantic, the Central American Isthmus
closed, abundant phosphate deposition ceased,
many organisms, vertebrate and invertebrate,
became extinct. Florida lay, as it still does, at the
crossroads of distinct biological and geological
provinces. If we can understand the geological
and biological responses of these dramatic
changes in a relatively young geological section,
Swe may be able to apply that understanding to
similar changes farther down in the record.

The papers in this volume represent a first
attempt to address some of these issues. They
include studies from a wide diversity of points of
view, from paleobiology to hydrogeology to
sequence stratigraphy. We hope that they
represent a new phase in the geological
exploration of Florida, one that rejects the
fragmentation of effort and isolation of individual

studies from other areas of geology, and toward
integrative approaches that make use of all
available data. If this is the case, then we may
yet look forward to the time when we no longer
have to say that we know "surprisingly little" about
the recent geological history of America's tropical
peninsula.

This volume is a revision of Guidebook
Number 31 of the Southeastern Geological
Society's annual fieldtrip, which took place in
Sarasota and Manatee Counties December 7-8,
1990.

Several papers that appeared in the
guidebook (those by Meeder and Waldrop and
Wilson) are not in the present volume, while the
paper by Muhs et al. in this volume was not in the
Guidebook. All papers that appeared in the
Guidebook have been revised by their respective
authors for publication here.

We would like to acknowledge the able
assistance of a number of individuals who aided
in the preparation and editing of this volume.
Cindy Collier corrected all the edited manuscripts
and formatted the text for publication. Jim Jones
and Ted Kiper corrected figures and provided
negatives for all figures. Ken Campbell, Jon
Arthur, Frank Rupert and Jacqueline Uoyd
provided editorial assistance.

FIGURE 1. Highly schematic stratigraphic section for the Plio-Pleistocene of southern Florida, Indicating
principal stratigraphic units traditionally recognized. Hiatuses separate all units but are not
indicated. Cross-hatched units are composed of variously fossiliferous sands, clays and
limestones, but also contain major shell beds.

Stratigraphic unit Location

Age Diversity Reference

Waccamaw

Pinecrest

Chipola

Moody's Branch

Gosport

Cook Mountain

"Facies Charrie"

N,S Carolina Pleist.

Florida

Florida

>500? Campbell,
et al. (1975)

Pliocene >1200? Olsson (1968)

Miocene >1000 Vokes (1989)

Mississippi Eocene

Alabama

326 Dockery (1977)

Eocene 483 Palmer and
Brann (1965-66)

Gulf Coast Eocene

France

Eocene

580 Dockery (1986)

700 Dolin and
Dolin (1980)

TABLE CAPTION

TABLE 1. Comparison of diversity (number of bivalve and gastropod species) of the Pinecrest molluscan
fauna of southern Florida with those of some other very high diversity faunas.

FLORIDA GEOLOGICAL SURVEY

SHELL BEDS A STATEMENT OF THE
PROBLEM

As pointed out by Kidwell et al. (1986), fossil
concentrations (= "shell beds") result from three
kinds of factors: biological, sedimentological and
diagenetic. In other words, highly fossiliferous
sediments are formed by some combination of
high densities of living organisms, physical
accumulation of organisms or their body parts
before or after death, low abiogenic sediment
input, and concentration of biogenic hardparts
after burial by compaction, selective dissolution,
or other diagenetic processes. In seeking the
origin of any shell bed, we may therefore break
the search for causal factors into the search for
biological, physical or sedimentological, and
diagenetic factors.

Kidwell et al. (1986) have suggested that
four observable aspects of shell beds are of
greatest taphonomic significance: taxonomic
composition, fabric or packing, bed geometry,
and internal structure or complexity. Geometry,
or the overall shape and extent of the bed, is
perhaps the first, most obvious aspect. Within
whatever geometry the bed displays, the other
three features can be examined. My own
approach to this small-scale taphonomic analysis
of the Pinecrest shell beds concentrates on their
dissection into what may be called "genetic units",
a flexible utilitarian notion of the stratigraphic-
geographic packages of fossiliferous sediment
resulting from one set of depositional and/or
taphonomic processes or events, as indicated by
their taxonomic composition and fabric.

In shell pits in northern Sarasota County
(Figure 2), for example, the Pinecrest section was
divided into 12 numbered units by Petuch (1982),
and this system has been refined by Stanley
(1991, pers. comm.). Recognition of genetic units
begins with these beds (Figure 3), and they are
broken down further as required. The Pinecrest
at Sarasota, as a whole, is thus an "internally
complex" shell bed, sensu Kidwell et al. (1986).
Within it, however, further levels of complexity can
be explored. Simple beds, those showing only a
single taphonomic signature, represent one
"genetic unit"; more heterogenous beds represent
"physical amalgamation of discrete shell horizons
into larger-scale, internally complex beds"
(Kidwell, 1986, p. 9).

The following discussion summarizes
available information on aspects of the Pinecrest
at small and large scales, and applies this
information to making some tentative statements
about the relative contribution of these causal
factors.

TAXONOMIC COMPOSITION

Petuch's (1982) units are based largely on
their constituent faunas. A bed-by-bed analysis of
the total faunas in these beds has never been
published, but even casual examination reveals at
least two conspicuous features: substantial
heterogeneity in faunal composition and frequent
dominance by one or a few taxa. The total
taxonomic diversity of the fauna remains
unknown. Following a detailed study of the
bivalves, Stanley (1986) believes that
approximately 220 species are present throughout
the entire Pinecrest. The gastropods have never
been studied comprehensively. Based on sorting
of bulk collections into identified and unidentified
morphospecies, between 500 and 600 species of
gastropods (Allmon et al., 1992) may be present
in the Pinecrest at Sarasota, perhaps three times
Stanley's number of bivalves. Olsson (1968),
however, suggested that as many as 1200
species of mollusks may exist in what he
recognized as the Pinecrest. If this is roughly
correct, and the bivalve: gastropod ratio seen at
Sarasota holds for the entire unit, one might
expect as many as 800 species of gastropods in
the Pinecrest throughout its full extent. It must be
emphasized that these are only rough estimates;
a thorough monographic treatment of the
Pinecrest gastropods is sorely needed.

A complex sequence of micro-environments
is represented in the Pinecrest at Sarasota,
reflecting change in physical environmental
conditions, biological productivity, community
structure, or all three. A substantial amount of
time-averaging (sensu Fursich, 1978; Fursich and
Aberhan, 1990) was involved in the formation of
these deposits (e.g., Geary and Allmon, 1990),
although the magnitude of averaging effects was
probably not constant throughout the total
temporal scope of the bed. The very high
species diversity of the Pinecrest therefore does
not represent a single community. The actual
alpha diversity of the Pinecrest fauna at any given
moment, however, can only be determined by the

SPECIAL PUBLICATION NO. 36

FIGURE 2. Map showing location of shell borrow pits in northern Sarasota County where the "Plnecrest
Beds" discussed here are well exposed.

FLORIDA GEOLOGICAL SURVEY

kind of careful taphonomic analysis that has yet
to be carried out.

GEOMETRY

Lateral extent and continuity

Despite their demonstrably wide extent and
conspicuous local stratigraphic manifestation,
total volumetric and areal extent of most south
Florida Plio-Pleistocene shell beds are unknown.
The Pinecrest has been recognized in pits or
excavation spoil piles from Sarasota to Miami, but
it may or may not be developed continuously
between known occurrences. Hunter (1978:83)
gives a "semi-diagrammatic" north-south cross
section along the northwest shore of Lake
Okeechobee that indicates the Pinecrest is
continuous at a thickness of around 40 feet over
perhaps 20 miles. DuBar (1958a,b) and Hunter
(1978) suggest that parts or all of the
Caloosahatchee/lower Fort Thompson shell beds
are thin and discontinuous across their outcrop
area.

Variation in thickness

Data from surface exposures and cores and
auger holes around the two active pits near
Sarasota indicate that the thickness of the
Pinecrest shell bed varies considerably on a scale
of kilometers from less than 1-2 m (3.3-6.6 ft.) to
more than 25 m (82 ft.) (Figure 4). Such variation
suggests significant relief on the underlying
depositional surface, in this case the sediments of
the Miocene-Pliocene Hawthorn Group. Such
relief could be a result of either paleodrainages or
paleokarst on the Hawthorn landscape. In the
cross section just mentioned, Hunter (1978)
indicates a "buried river channel" in the top of the
Bayshore Clay Member, apparently filled by
Pinecrest sediment. Missimer (1978) and Meeder
and Hunter (1983 -- an unpublished report cited
in Meeder, 1987) state that the upper surface of
the Hawthorn in Lee County is an irregular karst
plane. The probable timing of karst formation,
furthermore, is consistent with an important role
in controlling depositional surface topography;
Upchurch (1989) has suggested that much or
most of the subsurface karst in Florida formed in
association with the Messinian low stand of the
latest Miocene.

The topography of the underlying deposi-
tional surface is important for understanding the
formation of the Pinecrest. An irregular surface
would have allowed the shell beds to form in a
variety of depths below sea level at the same
distance from shore. Transgressive deposition in
paleo drainages may have been subject to greater
terrestrial influence. Steeper underlying
topographies might have had greater potential for
physical mechanisms of bioclast accumulation.

FABRIC

Bioclastic fabric refers to the orientation,
packing and degree of articulation displayed by
hardparts in a fossil deposit (Kidwell et al., 1986).
The fabric of the Pinecrest as a sediment is
)mostly bioclast-supported; that is, most of the
fossils are in contact with other fossils. Individual
beds, however, show wide variation in other
aspects of fabric. Some are best interpreted as in
life-position and others as physically accumulated
(Figure 5). Most fabrics fall in between these two
extremes, and it remains to document their
complete vertical and horizontal distribution. The
most important observation here may be that a
variety of fabrics is present, even within single
beds.

Fabric is one of the major criteria used in
interpreting a bed of fragmented branching coral
(Septastrea crassa) in unit 11 at Sarasota as a
probable tempestite (Ketcher and Allmon, 1992).
The complete depositional history of this bed is
complex, as evidenced by patterns of wear, size
and bioerosion (see Ketcher, this volume).

BIOEROSION/ENCRUSTATION

A class of observations perhaps intermediate
between taxonomic composition and fabric, with
implications for both, is the degree of bioerosion
and encrustation on shells, particularly those of
presumably infaunal species. These features
should be directly related to the length of time
shells lie exposed on the ocean floor, and so
should be useful indicators of hydraulic reworking
and/or shell bed formation in low sedimentation
rates.

Geary and Allmon (1990) used degree of
development of epibionts, and particularly the
ontogenetic patterns in encrustation, on shells of
Strombus floridanus to reconstruct the tapho-

SPECIAL PUBLICATION NO. 36

FIGURE 3.
A. Petuch's schematic column for the "Pinecrest" as exposed in the APAC pit near Sarasota, showing
12 numbered units (from Petuch, 1982).
B. Schematic N-S cross-section through the Pinecrest as exposed in the APAC and Quality Aggregates
pits near Sarasota. This diagram is a generalized cartoon intended to illustrate principal stratigraphic and
facies relationships observed in these pits, rather than to represent actual sections in detail. Horizontal and
vertical scales are only approximate. The diagram is based on the scheme of Petuch (1982; Figure 3A),
modified by Stanley (pers. comm. and in preparation) and my own observations.
Unit 11 is a muddy to silty gray to brown shelly sand containing bone fragments, and dense beds of
large barnacles, branching corals and encrusting cheilostomes (cf., Ketcher, this volume; Ketcher and
Allmon, 1990). It varies from 1-2 m in thickness. It is usually overlain by bed 10, characterized by abundant
large Mercenaria, especially the distinctive "tridacnoides" form. This unit is sometimes channelled away and
represented only by reworked Mercenaria or by a lag of blackened shell fragments. A probably brief hiatus
separates unit 10 from the overlying beds. Unit 10 is most often overlain by a bed of the lucinid bivalve
Anodontia alba, which is overlain by a bed of Vermicularia in mostly life position. Less often, unit 10 is
overlain directly by a bed of the oyster Hyotissa haitensis (unit 9), which is in turn overlain by Vermicularia
(unit 8). The Vermicularia bed seems to be simpler and thinner to the west and north and thicker and more
complex (i.e., consisting of 2-3 layers of Vermicularia and serpulid worm tubes, interbedded with Anodontia)
to the south and east (Figure 5). Vermicularia and Hyotissa are both seen to pinch out and exhibit locally
patchy distributions over wide areas of continuous exposure.
The Vermicularia bed usually is overlain by a concentration of the gastropod Strombus floridanus (see
Geary and Allmon, 1990; Figure 6). Less often, Hyotissa directly overlies Vermicularia without any strombids.
A bed of Hyotissa may or may not be developed over the strombid bed. At one locality at Quality
Aggregates, the strombids overlie the Mercenaria bed (unit 10) directly.
The strombid bed marks the bottom of unit 7 in Petuch's scheme, although the Anodontia bed may
also be part of this fauna. Unit 7 is the thickest unit in the sequence, and contains the most diverse fauna.
It lacks conspicuous Internal structure, but may change its sedimentological and faunal character upsection.
Hyotissa may or may not be present at the top of unit 7, in which case unit 6 is recognized. Unit 5 is an
upper Vermicularia layer, which was previously apparently continuous and conspicuous enough in the APAC
pit to be recognized as a discrete zone by Petuch and Stanley, but which is now represented only by small,
widely scattered clumps.
Unit 4, also known as the "black layer", is an organic-rich, muddy sand containing a brackish molluscan
fauna and abundant small terrestrial vertebrate material including wading and shore birds (e.g., Emslie, this
volume). It varies from 0 to 1 m thick. Unit 4 is apparently separated from underlying beds by an
unconformity, although it is often extremely difficult to discern. The evidence for a hiatus at this
unconformity includes: 1) age determination of the mammal fauna in unit 4 (Jones et al., 1991; Jones, this
volume), 2) age determinations of ostracode faunas, indicating that units 1-4 are considerably younger than
units 5-10 (Jones et al., 1991; Kamiya and Allmon, 1990, in prep.), 3) burrowing at the unit 4-6 contact at
least one locality (pers. obs.), 4) indurated, possibly calichified, horizons at or near the top of unit 6 (pers.
obs., W.B.Harris, pers. comm.).
It is usually overlain by a very sandy, often sparsely fossiliferous bed, unit 3. This bed contains
abundant mytilid bivalves (Perna sp.), which may be closely-packed in life-position. Another Hyotissa oyster
bed, unit 2, is often present above unit 3. Above and interfingering laterally with unit 2 is a zone of Pinecrest
shell that does not contain abundant, large Hyotissa valves. This can be referred to as "unit 2A". Above unit
2 is an irregular zone 1-2 feet thick of indurated, gray shelly sand, above which characteristic Pinecrest
molluscan taxa are not found. Petuch's unit 1 lies above this zone, and contains characteristic
Caloosahatchee guide fossils (see Lyons, this volume, 1991). The upper complex of units at the APAC
quarry (= Petuch's units 1 and 0, in part) contains a series of apparent paleosols and peat units, indicating
multiple transgressive-regressive episodes and containing molluscan shell beds as young as Ft. Thompson
age.

FIGURE 4. Isopach map showing thickness of shell in the area of the APAC and Quality Aggregates
pits near Sarasota, based on personal observations in pits and data from auger and core
holes kindly provided by Quality Aggregates, Inc. Thicknesses are in feet. Open squares
= representative thicknesses observed in quarry walls; solid squares = cores; solid circles
= auger holes.

FLORIDA GEOLOGICAL SURVEY

B.

FIGURE 5. Two examples of shell fabrics and orientations indicative of life position, surrounded by
fabrics and orientations suggestive of at least some degree of transport.

A. Bed ofAnodontia alba, a deeply infaunal lucinid bivalve, which is often found in layers or pods below
the lowermost Vermicularia layer ("bed 8"; see Figure 3). Photo taken on the west wall of the Phase 1 pit,
Quality Aggregates.

nomic history of a zone dominated by the species
in lower unit 7 (Figure 6). Most shells were clean
of encrusters or borers, suggesting that they did
not lie exposed on the bottom for very long. Yet
the density of shells in the layer was much higher
than observed in any living strombid species.
Geary and Allmon therefore concluded that these
gastropods were concentrated by repeated
episodes of rapid burial, followed by removal of
most of the sediment by winnowing, probably due
to storms.

Darrell and Taylor (1989) have recently
described the occurrence in the Pinecrest of an
encrusting scleractinian (probably Septastrea
marylandica) on hermit crab-inhabited gastropod
shells (Figure 7). Although many examples of
hermit crab-epibiont associations are known (e.g.,
bryozoans, anemones, sponges), this is the only
known example of such a relationship with a
scleractinian. In the Pinecrest, this coral species
occurs only in an encrusting form and only on
gastropod shells, almost all of which appear to
have been inhabited by hermit crabs (as indicated
principally by the coral growth form, an
associated cheilostome Hippoporidra, and the
ichnogenus Helicotaphrichnus; Darrell and Taylor,
1989; Schellenberg and Allmon, 1991).

The beds in which these coral-encrusted
shells are found contain abundant bivalve shells,
almost none of which are encrusted by this coral
(the only exceptions are a few pectinids). This
pattern suggests that, by carrying the gastropod
shells over the substrate surface, hermit crabs
may have prevented them from being buried, an
interpretation consistent with experimental
evidence from Recent hermit crab-epibiont
associations (Conover, 1975). The pectens were
mobile to some degree and so could also have
avoided burial. Beds with higher abundance may
have been more affected by at least episodically
high rates of sedimentation followed by storm
winnowing of sediment to concentrate the shells.

Vermeij (1987) has suggested that hermit
crab-inhabited, epibiont-encrusted shells are most
common today in deeper waters, where the
supply to shells is lower (the epibionts often
expand the size of the host shell, presumably
allowing the hermit crab to inhabit it for a longer
period). If this was true for the coral-encrusted
shells in the Pinecrest, then their high abundance
could indicate 1) a deep environment for deposi-

tion of the unit, 2) transport of the encrusted
shells from deep water by currents or storms, or
3) a shallow-water habitat for the encrusted shells
distinct from that typical of modern hermit crab-
epibiont associations. Preliminary data
(Schellenberg and Allmon, 1991, in prep.) indicate
that abundance of both complete coral-encrusted
shells and fragments vary in the Pinecrest by as
much as two orders of magnitude. Clearly one or
a combination of environmental (and therefore
perhaps taphonomic) factors was varying during
Pinecrest deposition.

SEDIMENTOLOGICAL DATA

Detailed sedimentological studies apparently
have never been carried out on the Pinecrest
section. Preliminary results from paleoecological
analysis of ostracodes (Kamiya and Allmon, 1990,
in prep.) indicates that smaller ostracodes and
finer sediments are more abundant inside arti-
culated bivalves than outside. A more detailed
survey of sedimentological patterns in the
Pinecrest at Sarasota (Nocita and Allmon, 1991,
in prep.) indicates that mud content of the
sediments is very low (usually < 5%). Similar
results were obtained by Meeder (1987). These
data are all consistent with a significant role for
sediment winnowing in the formation of the
Pinecrest beds.

Granulometric analyses of the main shelly
unit in the Pinecrest at Sarasota, unit 7/6 (see
Figure 3) (Nocita and Allmon, 1991, in prep.)
indicates a fining-upward sequence in both the
carbonate and non-carbonate sand- and mud-size
fractions between the shells. This is difficult to
explain as the result of storm deposition, but
could be an overprint on storm deposits (formed
by one of several processes; see below) of
changing facies associated with change in depth.

UNCONFORMITIES

In addition to analysis of individual "genetic
units" the distribution of stratigraphic hiatuses
within the Pinecrest section is of considerable
taphonomic importance. This importance lies
chiefly in the total temporal scope of the shell
beds, i.e., the length of time over which they
accumulated. There is currently some
disagreement as to the age and temporal scope
of the Pinecrest beds at Sarasota (e.g., Jones et
al., 1991; Stanley, 1991; Jones, this volume). If

FLORIDA GEOLOGICAL SURVEY

FIGURE 6. Concentration of Strombus floridanus in the lower portion of "bed 7" (see Figure 3); west
wall, south pit, APAC (see Geary and Allmon, 1990, for more details).

these shell beds accumulated in 104 105 years a
different set of taphonomic processes may have
been involved than if it took 10s 106 years. The
recognition of stratigraphic hiatuses can
contribute to resolving this issue.

Petuch (1982) recognized eight disconfor-
mities in the Pinecrest at APAC (see Figure 3):
between units 11 and 10, 10 and 9, 9 and 8, 8
and 7, 6 and 5, 5 and 4, 3 and 2, and 2 and 1.
Stanley (1991, pers. comm.) believes that hiatuses
separate only units 11 and 10, 10 and 9, and 1
and 0, and that the remainder (units 9-1) formed
during a single transgressive-regressive episode.
A hiatus below unit 4 is supported by
biostratigraphic and other data (see Figure 3); if
confirmed, it would mean that the total Pinecrest
section at Sarasota accumulated in closer to 105 -
106, than to 104 years.

BIOLOGICAL VERSUS SEDIMENTOLOGICAL
FACTORS

The Plio-Pleistocene shell beds of southern
Florida were formed by some combination of
sedimentological, biological and diagenetic
processes. If we, as a provisional simplifying
assumption, ignore (or assume as constant)
diagenesis, then some combination of sedimen-
tological and biological factors was involved. It is
then logical to ask whether similar shell beds are
forming today on the west Florida shelf. No
explicit or detailed studies have ever addressed
this issue. For the time being the answer appears
to be no (pers. obs.; Lyons, pers. comm.; see
Lyons, 1979; Moore, 1980). If this is indeed true,
then either sedimentological or biological, or both,
conditions have changed.

Sedimentological Factors

Many bioclastic fabrics observed in the
Pinecrest and other densely shelly Plio-
Pleistocene units are suggestive of storm
deposition (e.g., Geary and Allmon, 1990; Figure
5). Many of these beds, however, show bioclastic
fabrics, orientations and levels of encrustation
indicative of burial either in "life position" or after
minimal exposure and transport. In many depo-
sits, a significant proportion of the shells shows
little or no surface abrasion. If environmental
energy has been an important factor in concen-
trating these shells, it apparently acted sometimes
with substantial energy, affecting significant

transport, and at other times without moving the
shells long distances laterally over the substrate.

To the degree (thus far undetermined) that
storms have been important in the formation of
these beds, at least two different storm-influenced
depositional scenarios are possible. One is
similar to that discussed by Brackett and Bush
(1986), based on Recent deposits on the north
coast of Puerto Rico. They describe a coarse
basal shell lag overlain by finer deposits that
settled out after passage of the storm. Some of
the Pinecrest shell beds could be analogous to
such a basal lag, with the overlying finer layers
stripped off by subsequent storm events. The
entire 2-3 m thickness of unit 7/6, for example,
could be a result of repetition of such depositional
episodes.

Alternatively, an episodic winnowing model,
similar to that proposed to explain a single bed
within the Pinecrest at Sarasota by Geary and
Allmon (1990), may be applicable to a large pro-
portion of these shelly units. In such a model,
biogenic hardparts would be buried relatively
quickly by high "background" rates of sedimenta-
tion. Since much of the Neogene plastic sedi-
ment on the west Florida shelf may be relict or
palimpsest (Holmes and Evans, 1963; Scholl,
1963; cf., Swift et al., 1971), apparently high local
sedimentation rates may have resulted more from
shifting or longshore transport of sand in waves
or tides than from deposition of fresh terrigenous
material. Johnson (1957) suggested that shells
could be buried by migrating ripples in this way
(see also Sternberg, 1967, 1972; Tedrick, 1972).
Following burial, storms would winnow away
much of the sediment, but without transporting
the shells very far. Similar suggestions have been
made by Gernant (1970), Westrop (1986), and
Beckvar and Kidwell (1988). Figueiredo et al.
(1982) consider several long- and short-term
processes for the formation of graded beds
during storms, and give particular attention to a
"bottom liquefaction" model in which sediments
are winnowed away and coarser clasts settle in
roughly their original area of deposition.

The Pliocene coasts of Florida may have
experienced more hurricanes given the slightly
warmer global climate during at least part of the
epoch (cf., Hobgood and Cerveny, 1988; Barron,
1989). Holocene records in Florida indicate strong
hurricanes affecting the southern coasts

FLORIDA GEOLOGICAL SURVEY

approximately every 10 years (Ball et al., 1967;
Galli, 1989). This high frequency might have a
very significant effect on the sedimentological
record of the area, and an increase in intensity
obviously even more so.

Biological Factors

The strictly geological processes of deposi-
tion and redeposition of relict or palimpsest
plastic and carbonate sediments in a regime of
fluctuating sea levels on a broad, shallow shelf
have not changed on at least the west coast of
Florida since the Late Miocene (e.g., Hine et al.,
1988). This raises the possibility that biological
processes were more important than sedimento-
logical and biostratinomic processes in the gene-
sis of these shell beds. Biological productivity
seems a particularly likely candidate for such a
process.

Although it is very difficult to demonstrate
conclusively, it is possible that levels of pro-
ductivity were higher off some or all of the Florida
coast in the Plio-Pleistocene than they are today.
Circumstantial evidence consistent with the hypo-
thesis that upwellings of cooler, perhaps nutrient-
rich waters occurred during the formation of at
least the Pinecrest shell beds includes the
following:

1. Modern upwelling. Austin and Jones
(1974) report seasonally high standing crops of
plankton in waters over the Florida Middle
Grounds in the northeastern Gulf, possibly
associated with up-welling of cooler, higher-
salinity waters. The west coast of Florida is not
usually thought of as an area of active upwelling
today (unlike the east coast; e.g., Smith, 1982),
but the plankton densities reported by Austin and
Jones for seasonal highs approach or exceed
values for many well-documented areas of
upwelling elsewhere in the world. Of course the
existence of upwelling today is no guarantee of
upwelling in the past, but it may indicate that such
oceanographic patterns are possible.

2. Temperature. From extensive Recent
data, Cronin and Dowsett (1990) have con-
structed a mathematical transfer function for the
relationship between ostracode assemblages and
temperature. Based on ostracode samples from
the Pinecrest beds at Sarasota, they conclude
that bottom temperatures during deposition were

no warmer and as much as 2.4 C cooler in
August and 0.6 *C cooler in February than
present values.

Cooler temperatures may also be indicated
by the almost total lack of calcareous and
coralline algae in Pinecrest sediments at Sarasota.
Both are abundant at this latitude in the eastern
Gulf of Mexico today (Taylor, 1960; pers. obs.).
Since temperature is widely believed to be a
primary control of benthic algal distribution (e.g.,
van den Hoek, 1975; Lawson, 1978), a tempera-
ture change may have been responsible for this
floral shift since the Pliocene. Meeder (1987)
notes that the carbonate sediments of the
Tamiami Formation can be divided, using the
terms of Lees (1975), into foramol (chiefly
foraminifera and mollusks) and chlorozoan (algae
and coral). The mollusk-dominated sediments of
the Pinecrest at Sarasota would fall under
foramol. Foramol sediments are generally held to
form in cooler waters than chlorozoan sediments
(Lees, 1975).

The Pinecrest at Sarasota (specifically beds
6 and 7 of Petuch; see Figure 3) contains several
molluscan taxa whose modern representatives in
the eastern Gulf of Mexico today are most
common at depths of 20-30 m (W.G.Lyons, pers.
comm.). These include Bullata taylori (Olsson)
(B. bullata occurs today at 100-150 feet off Brazil;
Lyons, pers. comm.), Sconsia hodgeii (Conrad)
(S. striata lives in 50-255 fathoms in the Gulf;
Abbott, 1974), and Scaphella floridana (Heilprin)
(S. junonia is most abundant in about 30 m in the
Gulf; Abbott, 1974). These taxa could be indi-
cators of similar depths of formation for at least
this bed of the Pinecrest at Sarasota.

One of the most common bivalves in the
Pinecrest at Sarasota is Argopecten eboreus
(Conrad), which sometimes dominates beds and
shows current-imbrication. Paleoenvironmental
interpretation of these pecten beds is hindered by
lack of a clear modern analog for the extinct A.
eboreus. Morphologically, it is similar to both the
Recent bay scallop, A. irradians (Lamarck), and to
the smaller calico scallop, A. gibbus (Linnaeus).
A. irradians is most common in shallow water (0-
20m), while A. gibbus is occurs down to 400m
(Abbott, 1974). Based on an analogy with A.
gibbus, Waller (1969:60) suggested that A.
eboreus was "an open-water scallop, perhaps
preferring deep, open embayments with some-

SPECIAL PUBLICATION NO. 36

what restricted bottom circulation". If, however,
closer analogy with A. irradians is drawn, then the
environment of A. eboreus might be interpreted
as shallow. DuBar and Taylor (1962), for
example, concluded that A. eboreus in the
Jackson Bluff Formation of northern Florida lived
in no more than 10 m of water.

Depths for the Pinecrest greater than a few
meters might be consistent with the storm-
influenced depositional scenarios described
above, since modern storms are known to
transport sediment at depths as great as 40-50 m
(Hayes, 1967; Lavelle et al., 1978; Highsmith et
al., 1980; Gagan et al., 1990; Miller et al., 1990).
It is also consistent with recent reconstructions of
Pliocene sea levels on the coastal plain as high as
30 m above present mean sea level (Dowsett and
Cronin, 1990).

Apparently "deep water" mollusks, however,
could also be indicative of temperature change.
If temperatures were slightly cooler (or if cooler,
deeper waters periodically flooded the shelf)
during Pinecrest time, these taxa may have
inhabited shallower depths than they do now, and
may have retreated to their current greater depth
preferences at some time since the end of the
Pliocene.

3. Corals. 46 species of corals are known
from the Pinecrest Beds (Weisbord, 1974; Stanley,
1986). Among the most common are Septastrea
marylandica, which occurs mostly as
encrustations on gastropod shells (see above), S.
crassa, which occurs as branching colonies in a
dense bed (Ketcher and Allmon, 1992; see
above), Oculina sarasotana, which occurs as
branching colonies on Hyotissa oyster biostromes
in units 7 and 2 (see Figure 3), and Solenastrea
spp., which occurs as massive heads up to 80 cm
in diameter. Solenastrea is facultatively
zooxanthellate today off North Carolina, and
prefers cooler, more turbid waters (W. Jaap, per.
comm.). The genus Septastrea is extinct (and so
we do not know whether it was zooxanthellate or
not); living species of Oculina are both
zooxanthellate and azooxanthellate, living in both
deep and shallow waters (Squires, 1958; Reed,
1980, 1981, 1983).

4. Turritella beds. The Cenozoic record of
Florida is peculiar for its paucity of beds
dominated by the gastropod Turritella s.I., which

are common features of the Cenozoic column
throughout the remainder of the U.S. Gulf and
Atlantic coastal plains (Allmon, 1988a). This is
interesting in the present context in that turritellid-
dominated communities often occur today in
areas of upwelling (Allmon, 1988b). In the
Pinecrest section exposed at Sarasota, however,
are at least two beds (in upper unit 7 and unit 1;
see Figure 3) dominated by two different species
of Turritella. Analysis of these beds is not yet
complete, but they may indicate at least a local
change in the nutrient regime (Spizuco and
Allmon, 1992).

5. Indirect evidence of upwelling. Stanley
(1986) has suggested that upwelling occurred
along the west coast of Florida during the
Pliocene. He bases this suggestion on both the
abundance and diversity of mollusks in the
Pinecrest and on purported biogeographic
division between the northern Caloosahatchian
(Virginia south to Florida) and southern Gatunian
Provinces (most of the Caribbean and Central
America).

BROADER IMPLICATIONS

Changing levels of biological productivity
may have broader implications for how dense
fossil concentrations form. A crucial insight into
the formation of fossiliferous sedimentary deposits
was that the abundance of fossils in such depo-
sits is a function of relative rates of input of
biogenic skeletal hardparts and abiogenic sedi-
ments (Johnson, 1960; Kidwell, 1986). At the
simplest level, fossiliferous sediments result when
net hardpart input is high relative to net sediment
input. Shell beds would, thus, be expected to
result when net sedimentation rates were relatively
very low (Kidwell, 1986; Kidwell et al. 1986).
Kidwell (1986) has, in fact, claimed that changes
in rate of sedimentation are more important than
changes in hardpart input in controlling the
occurrence of fossil concentrations.

This scenario is complicated, however, by
the suggestion that rates of dissolution of
calcareous hardparts exposed at or near the
sediment-water interface are much higher than
most rates of shell productivity (Davies et al.,
1989; Powell et al., 1989). To the extent that this
is true, shell beds cannot form by relatively
gradual accumulation of hardparts in regimes of

FLORIDA GEOLOGICAL SURVEY

low sedimentation rates. For preservation to
occur, burial must be relatively rapid.

Kidwell (1989:16), however, lists several
reasons for believing that shells may not always
dissolve quickly, and so may, at least under
certain circumstances, accumulate gradually.

b) Winnowing of sediments increases poro-
sity and so exchange with overlying seawater,
which may be oversaturated with respect to car-
bonate. Concentrated shells may also contribute
to a buffered chemical microenvironment by dis-
solving and increasing levels of carbonate in
porewaters.

c) "Episodic exposure of shells and their
progressive accumulation within surficial sedi-
ments creates a favorable habitat for colonization
by typically larger-bodied, epifaunal suspension-
feeding organisms. These benthos not only con-
tribute hardparts to the initial concentration, but
further decrease its erodibility and seal off some
shells from destructive agents".

d) It has been shown experimentally that
shell destruction in marine environments is size
dependent, with highest rates at smaller sizes.
Since larger-sized individuals tend to be the most
important and persistent components of benthic
communities, "sand substrata that favor
colonization might therefore from the very outset
have a higher likelihood of yielding a preservable
condensed shell deposit."

e) Existence and apparently long persistence
of shells on Recent ocean bottoms "indicates that
skeletal material can survive even continuous
exposures on the seafloor longer than experi-
mentally determined half-lives would suggest, and
that shells are also more durable to repeated
cycles of exhumation and burial than would be
predicted from lab simulations". Possible
explanations for this persistence, Kidwell
suggests, might include: i) "very early diagenetic
stabilization" of the shells, ii) "ionic poisoning of

the shell surface that changes the kinetics of
dissolution", and iii) "protection of dead shells by
organic chelates, or by a 'slime coat' of algae or
bacteria".

"These observations," Kidwell concludes,
"suggest that hard-parts in concentrations have
greater potential for preservation than hardparts
that are sparsely dispersed" (1986, p. 16). Much
of the acidity that causes carbonate dissolution in
the TAZ, however, comes from degradation of
organic carbon (CO, HNO3, HPO,; Davies et al.,
1989, p. 208). In cases of very high biogenic
hardpart production, high amounts of organic
carbon might be expected to enter the sediment,
and increase, rather than decrease acidity and so
decomposition (Davies, pers. comm.).

These two views of possible mechanisms for
shell bed accumulation have very different impli-
cations for paleoenvironmental, paleoecological
and evolutionary conclusions that might in theory
be drawn from analysis of such beds. Beds can
accumulate gradually, at least under certain
circumstances, in which case there may be posi-
tive effects of time averaging, and "bias of the
general trend of ecological replacement in the
community is thus minimized" (Kidwell and
Behrensmeyer, 1988, p. 5). Or shell beds can
form through episodic, relatively sudden burial, in
which case stratigraphic acuity will be, on
average, much lower and so ecological processes
on short or even moderate timescales will be
normally inaccessible to the paleontologist (cf.,
Brandt Velbel, 1984). Careful analysis of a high
diversity, complex fossil shell bed such as the
Pinecrest may allow these two hypotheses to be
tested. Abundant evidence for episodic burial and
numerous, short-lived genetic units will support
the rapid burial view; evidence for accumulation
over longer intervals of time will support the
gradual view.

CONCLUSIONS

Answering the question "whence the Pine-
crest (or any other) shell bed?" in the Florida Plio-
Pleistocene depends on the scale at which one
wants to know the answer. The Kidwell et al.
(1986) trichotomous causal classification may be
applied at any of several scales. At the large end,
the question may be "why are there any large
shell concentrations at all?" or "why are they so
large?" The answers) may involve answering

Moore, D. R., 1980, The shallow water fauna of
Sanibel and its relationship to Upper Cenozoic
fossils in South Florida: in Gleason, P.J., ed.,
Water, oil and the geology of Collier, Lee and
Hendry Counties: Miami Geological Society Field
Trip Guidebook, p. 57-59.

Olsson, A. A., 1968, A review of Late Cenozoic
stratigraphy of southern Florida: in Perkins, R.D.,
ed., Late Cenozoic stratigraphy of southern
Florida a reappraisal: Second Annual Field Trip
of the Miami Geological Society, p. 66-82.

Palmer, KV.W. and Brann, D. C., 1965-1966,
Catalog of the Paleocene and Eocene Mollusca of
the southern and eastern United States: Bulletin
of American Paleontology, v. 48, 1057 p.

Petuch, E. J., 1982, Notes on the molluscan
paleoecology of the Pinecrest Beds at Sarasota,
Florida with the description of Pyruella, a
stratigraphically important new genus
(Gastropoda: Melongenidae): Proceedings of the
Academy of Natural Sciences of Philadelphia, v.
134, p. 12-30.

1981, In situ growth rates of the
scleractinian coral Oculina varicosa occurring
with zooxanthellae on 6-m reefs and without on
80-m banks: Proceedings of the 4th International
Coral Reef Symposium, v. 2, p. 201-206.

1983, Nearshore and shelf-edge Oculina
coral reefs: the effects of upwelling on coral
growth and on the associated faunal
communities: in Reaka, M.L., ed., The ecology of
deep and shallow coral reefs. NOAA Symposium
Series in Undersea Research, v. 1, p. 119-124.

__ 1991, Evidence from marine deposits
in Florida that the Great American Interchange of
mammals began prior to 3MA: Geological
Society of America, Annual Meeting, Abstracts
with Programs, v. 23, no. 5, p. A405.

Tedrick, P., 1972, Direction of longshore drift,
littoral current velocities, and sand migration
along the Pinellas County, Florida coastline from
1925 to 1960, with prediction of future trends:
[MS thesis], Tampa, Florida, University of South
Florida, 122 p.

COASTAL PLAINS STRATIGRAPHY:
THE DICHOTOMY OF BIOSTRATIGRAPHY AND UTHOSTRATIGRAPHY-
A PHILOSOPHICAL APPROACH TO AN OLD PROBLEM

Thomas M. Scott
Florida Geological Survey
Tallahassee, FL 32304-7700

INTRODUCTION

Recognizing formations in the coastal plains
sediments of the southeastern United States
remains a tenuous situation in the 1990's.
Formations described in this area often rely on
the biostratigraphic nature of the sediments to
identify them. Although this practice was the
acceptable mode of operation for much of this
century, current geologic thought defines for-
mations through their lithologic composition.
Paleontology is then utilized to describe the faunal
composition, the relative time frame and the
depositional environment of the sediments. The
North American Stratigraphic Code (North
American Commission on Stratigraphic Nomen-
clature [NACSN], 1983) provides the definitions of
the various units geologists use to describe the
sediments regardless of whether the units are
lithologic, faunal, seismic or other types. The
challenge we as geologists now face, is to under-
stand the sediments of the coastal plain in light of
the Code of Stratigraphic Nomenclature and to
define usable lithologic units.

Why do the problems of the dichotomous
nature of our coastal plains sediments exist? The
answer to this query is relatively straightforward
when one considers the way in which geologists
initially investigated the geology of the south-
eastern coastal plains- an area of few and
scattered exposures. A geologist visiting an area
for the first time, had to rely on paleontologic
knowledge to determine the age of the sediments;
thus the sediments were often referred to, for
example, as the "Orbitoides limestone" charac-
terized by Orbitoides mantelli (Dall and Harris,
1892). Knowledge of the lateral extent of the
sediments was generally not known due to low
land-surface relief and very little to no subsurface
data. In this way, outcrops scattered throughout
the coastal plain were placed in a comprehendible
sequence. However, this required that the

geologist must possess paleontologic knowledge
and apply that knowledge to the sediments at
hand. Much of the paleontologic knowledge
applied to these sediments related to the larger
microfossils and the invertebrate macrofossils,
often the mollusks. During the last fifty years, a
tremendous body of paleontologic data concern-
ing these and other fossils has been amassed.
From this information, the realization of the
problems associated with delineating formations
on the basis of their enclosed fossils became
apparent. The first stratigraphic code, which
eventually evolved into the modem Code of
Stratigraphic Nomenclature, arose in response to
the needs of geologists to avoid the confusion
that came about due to the recognition of units
based on the faunas.

UTHOSTRATIGRAPHY

The North American Stratigraphic Code
defines a formation as "...the fundamental unit of
lithostratigraphic classification. A formation is a
body of rock identified by lithic characteristics and
stratigraphic position; it is prevailingly but not
necessarily tabular and is mappable at the earth's
surface or traceable in the subsurface." By
definition, a formation cannot be recognized on
the basis of particular fossils being present or
absent within the sediments. Obviously, fossils
cannot be ignored since they comprise an impor-
tant component of the sediments. A formation
may be recognized on the basis of being a fossili-
ferous unit where the fossils are considered as
nothing more than grains forming part of the
lithology. The fossils are very important in that
their recognition may indicate the relative age of
the formation and aid in avoiding confusion with
similar sediments of differing ages.

The accurate recognition of lithostratigraphic
units is increasingly important as geology has
become more specialized. A geologist no longer

SPECIAL PUBUCATION NO. 36

has the "luxury" of being a general geologist. In
this day and age of specialization and the
information explosion, geologists, by necessity,
must concentrate their efforts in particular areas
and rely on the specialists in other areas to
provide assistance in those fields outside the
individual's expertise. The efforts of geologists,
as represented in the papers of this volume, are
an excellent example of the need to seek the
assistance of other specialists. For example,
Vacher et al. (this volume) provide an insight into
the problems encountered by hydrogeologists as
attempts are made to define the hydrologic re-
gime in terms of units characterized by particular
faunas.

To be of value, a coastal plain lithostra-
tigraphic unit must be mappable on a regional
scale, lithologically distinct from subjacent and
suprajacent units and recognizable in the sub-
surface. The last criteria, recognizable in the
subsurface, is of major importance in an area of
little to no significant topographic relief as is the
case over much of Florida. Subsurface samples,
whether they are cores or cuttings, rarely provide
the necessary macrofossil diversity to recognize
faunally-delineated units. In Florida, the vast
majority of stratigraphic investigations are
conducted through subsurface samples. These
investigations are often related to the hydro-
geology of an area or to potential environmental
hazards. Geologists conducting these investi-
gations generally do not have the luxury of
"figuring out" the stratigraphy from scratch. They
rely on other geologists to provide a coherent,
useful stratigraphic sequence defined in such a
way as to allow unit identification from the
subsurface samples obtained during a project. It
is up to those geologists who have the "luxury" of
conducting thorough, scientific investigations to
delineate the lithostratigraphic units in a
comprehendible and useful fashion. These units
must be defined on their areal extent, thickness
and a detailed lithologic description.

Geologists working in the coastal plains
sediments must recognize the necessity of the
lithologic characterization of lithostratigraphic
units. They need also accept the fact that the
knowledge of the stratigraphic sequence of an
area is not static but dynamic and evolves, by
necessity, as new data becomes available. In
order to achieve the goal of recognizing litho-
stratigraphic units, geologists must separate the

faunal zones from the lithologic units incor-
porating the fossils. To do this, paleontologists
working in the coastal plains sediments need to
refrain from incorrectly equating faunas to
formations, members, etc. and delineate the
biostratigraphic units which occur within litho-
stratigraphic units (see Lyons, this volume for a
comment). In order to build a proper strati-
graphic sequence, incorporating both lithologic
and faunal characteristics, geologists are going to
have to identify, for example, Formation X and
recognize biozones 1,2 and part of 3 as occurring
within Formation X. Geologists can no longer
accept stratigraphic discussions or descriptions
that refer to Formation X as defined by the
Formation X fauna. We must make the separation
of the two concepts. Only through this con-
ceptual change and the application of an inte-
grated stratigraphic approach (Jones, this
volume) can we construct a useful and usable
stratigraphic sequence which will fulfill the needs
of the geologic community. By accepting and
implementing these recommendations, geological
mapping projects, subsurface and hydrostra-
tigraphic investigations and biostratigraphic
research will all benefit and the coastal plains
geologic history will be more easily deciphered.

FLORIDA STRATIGRAPHY

In Florida, biostratigraphic means have been
used extensively to differentiate sediments in the
Cenozoic section. For example, the Paleocene
Cedar Keys Formation, the Eocene Oldsmar Lime-
stone, Lake City Limestone and Avon Park Lime-
stone were subdivided and named by Applin and
Applin (1944) on the basis of faunal elements
foraminiferaa). Purl (1957) raised the Ocala
Limestone to Group status and subdivided it on
the basis of the foraminifera. Mollusks have been
utilized extensively to recognize formations in
Florida. The units recognized in this manner
include the Pinecrest, Caloosahatchee, Bermont
and Fort Thompson. The Bermont formation
provides an excellent example of a unit defined
solely on faunal criteria. The Bermont formation,
as informally defined by DuBar (1974), was
separated from the Caloosahatchee Formation
even though the two units "cannot be distin-
guished readily by their lithologic characteristics".
The separation of the two units is based on
comparative faunal analysis."

FLORIDA GEOLOGICAL SURVEY

In recent years, there have been many inves-
tigations aimed at differentiating the sediments on
a lithologic rather than a paleontologic basis.
Miller (1986) formally suggested dropping the
name Lake City Limestone from use since it could
not be accurately and consistently recognized on
lithologic criteria. He recommended that the Lake
City Limestone be placed in the Avon Park For-
mation (changed from Limestone). The recog-
nition of Neogene units in the Georgia coastal
plain based on lithologic criteria was discussed by
Huddlestun (1988). Scott (1988) formally recog-
nized the Hawthorn sediments in Florida as a
group and subdivided them on the basis of litho-
logies. Other authors, including Hunter (1978)
and Missimer (1984), have recognized the lack of
compliance with the North American Stratigraphic
Code in the development of the Plio-Pleistocene
stratigraphic sequence in Florida. Missimer (this
volume) utilizes lithologic criteria to describe the
complexities of the Tamiami Formation in south-
western Florida. Hunter (1978) in an attempt to
delineate lithologic units in southern Florida,
informally suggested the incorporation of the
Caloosahatchee and Fort Thompson sediments
into a single lithologic unit.

Perkins (1977) described the Pleistocene
sequence in southern Florida as characterized by
similar lithologies separated by discontinuities
(Figure 1). Perkins further stated "Were it not for
these discontinuities, these sequences would
appear to represent depositional entities and
would be grouped under one formational desig-
nation." Based on the current stratigraphic code,
the separation of the Pleistocene rocks based on
the discontinuities, as done by Perkins (1977),
should be referred to as allostratigraphic units not
lithostratigraphic units (NACSN, 1983). In light of
the current code, Perkins' statement supports the
use of one formational entity for the entire
sequence.

Understanding the Plio-Pleistocene
sediments in Florida provides an interesting
challenge for coastal plain geologists.
Recognizing lithostratigraphic units within these
sediments requires the combined efforts of
paleontologists and lithostratigraphers. The
recognition of lithostratigraphic units in the Plio-
Pleistocene of southern Florida is necessary for
the completion of the new Geologic Map of
Florida currently being created by the Florida
Geological Survey. Field mapping conducted by

Survey geologists has reiterated the fact that
many of the "formational" entities in southern
Florida can not be mapped in a lithologic sense.
Such an effort, being undertaken by the authors
of several of the papers in this volume, Involves
the delineation of lithologic units and the
concurrent recognition of biostratigraphic zones.

This author, in cooperation with several
paleontologists and geologists, is developing the
conceptual framework of a lithostratigraphic unit
which includes the faunally-derived
Caloosahatchee, Bermont and Fort Thompson
"formations" (Figure 1). This unit, informally
referred to here as the Okeechobee formation,
extends over much of southern Florida (Figure 1).
The Okeechobee formation consists of variably
shelly siliciclastic and carbonate sediments that
may reach 100 feet thick in southeastern Florida.
Exposures of these sediments are generally
limited to a few dewatered shell pits that occur
widely scattered over the area. Most operations
mining the Okeechobee formation sediments do
not dewater and the sediments can be seen only
on spoil piles.

The Florida Geological Survey will drill
continuous cores at selected sites in southern
Florida to provide the necessary subsurface data
and type cores. If the present investigation
validates the concept of the Okeechobee
formation, the formational nomenclature will be
formalized in accordance with the NACSN (1983).

VICTOR A. ZULLO and W. BURLEIGH HARRIS
Department of Earth Sciences, University of North Carolina
at Wilmington, North Carolina 28403

INTRODUCTION

Sequence stratigraphic analysis has proven
to be a useful tool in unravelling complex
temporal and spatial faces relationships in the
marine environment where more traditional
stratigraphic approaches have failed. In areas of
limited exposure where facies changes are both
rapid and repetitive, normal mapping procedures
and the correlation of lithostratigraphic units are
virtually impossible. As a result, mapping and the
designation of units is often based on biostrati-
graphic criteria, and the resulting units are, in fact,
biostratigraphic zones. Although there is nothing
inherently wrong with such procedures, the
success of such practices is dependent on the
presence of age-diagnostic fossils throughout the
region mapped. Because of the vagaries of
preservation and the potential of many organisms
to be facies-dependent, widespread distribution of
age-diagnostic fossils is seldom realized.

Sequence stratigraphy, on the other hand,
is based on the recognition of depositional
sequences developed as a result of relative rise
and fall of sea level in coastal basins. These
depositional sequences are genetically related
packages of sediment bounded by unconformities
toward the basin margin or correlative
conformities in the basin. A complete
depositional sequence consists of sets of
lithologies (systems tracts) and surfaces whose
internal characteristics are determined by the
effects of relative rise and fall of sea level on
sediment deposition. Preservation of individual
systems tracts is further dependent on the
location of the study area within the basin of
deposition. Depositional sequences near the shelf
break tend to be preserved in their entirety,
whereas certain systems tracts and surfaces are
absent toward the basin margin (Figs. la, 1b).
The record of depositional sequences in the
Coastal Plain is most often a record of basin
margin deposits and, as a result, preserved

sequences are incomplete. Conversely, uncon-
formities bounding depositional sequences are
often best developed and most easily recognized
in these basin margin deposits.

As illustrated in Figures la and 1b, sediment
packages representing depositional sequences
near the basin margin tend to lack well-defined
condensed sections, and seldom include the
lowstand deposits developed above Type 1
unconformities, or the shelf margin deposits
above Type 2 unconformities. Type 1
unconformities develop when sea level falls below
the shelf break, whereas Type 2 unconformities
develop when sea level does not fall below the
shelf break. Sequences on basin margins usually
are represented by thin transgressive deposits
separated by a marine hiatus (surface of
maximum flooding) from thicker overlying
highstand deposits. In these cases the
transgressive surface is collapsed on the
underlying unconformity. In extreme updip areas,
highstand deposits unconformably overlie
sediments (also often highstand) of a previous
sequence. However, as regional differences in
sedimentation rates and accommodation also play
a role in the development and preservation of
systems tracts, the generalized distribution
patterns shown in Figures la and lb can only be
regarded as models.

This study has three goals. The first is to
develop a sequence stratigraphic model for the
exposed basin margin Pliocene and lower
Pleistocene of the Atlantic Coastal Plain, based on
well described sections in the Salisbury
embayment of Virginia and the Albemarle
embayment of North Carolina. The second is to
correlate the resulting model to the Global
Coastal Onlap Cycles proposed by Haq et al.
(1987). The third is to suggest a preliminary
application of the model to exposed Pliocene and
lower Pleistocene deposits in southern Florida,
and to outline future studies.

Figure lb. Type 2 sequence (sea level does not fall below shelf break), showing differences in signature
on the basin margin and within the basin. Note that the signature on the basin margin is-virtually identical
to that of a Type 1 sequence.

^

FLORIDA GEOLOGICAL SURVEY

A COASTAL ONLAP MODEL: THE PLIOCENE
AND LOWER PLEISTOCENE OF VIRGINIA AND
NORTH CAROLINA

The best documented and most complete
marine Pliocene-lower Pleistocene section on the
western Atlantic basin margin is in the Salisbury
and Albemarle embayments of the Virginia and
North Carolina Coastal Plain. Recent
lithostratigraphic (e.g., DuBar et al., 1974; Ward
and Blackwelder, 1980; Blackwelder, 1981; Ward,
1984, 1989) and biostratigraphic analyses (e.g.,
Akers and Koeppel, 1974; Hazel, 1983; Snyder et
al., 1983; Dowsett and Cronin, 1990) permit the
construction of a sequence stratigraphic model
with which the Plio-Pleistocene depositional
history of southern Florida can be compared.
The Pliocene-lower Pleistocene section in the
Salisbury-Albemarle embayments is as follows:

The Yorktown Formation disconformably
overlies the upper Miocene Eastover Formation.
Four members are recognized in the Yorktown
Formation. The basal Sunken Meadow Member
is a fossiliferous, glauconitic, phosphatic, coarse-
to medium-grained sand bounded by
unconformities. The overlying Rushmere Member
is a fossiliferous, phosphatic, glauconitic sand
with pebbles and coarse sand at its base. The
Rushmere Member grades upward into silt, clay
and fine sand of the Morgarts Beach Member.
The upper Moore House Member is composed of
fossiliferous sand, cross-bedded shell hash, and
bioclastic sand disconformably overlying the
Morgarts Beach Member. The Chowan River
Formation disconformably overlies the Yorktown

Formation and includes two members. The lower
Edenhouse Member consists of a discontinuous
basal sand containing pebbles and boulders to 1
m in diameter, and fossiliferous, bioturbated, silty
sand. The sand of the Edenhouse Member
grades upward into cross-bedded fine to medium
sand, interbedded silty sand, argillaceous silt, and
biofragmental sand of the Colerain Beach
Member. The James City Formation
disconformably overlies the Chowan River or
older formations, and consists of fossiliferous
argillaceous sand and sandy clay.

The Sunken Meadow Member of the
Yorktown Formation, bounded by unconformities
and characterized by glauconitic and phosphatic
coarse sand, represents only the transgressive
systems tract of a depositional sequence. This
member (equivalent to Zone 1 or the Placopecten
clintonius zone of the Yorktown Formation) is
correlated with the lowest part of Blow's (1969)
Planktonic Foraminiferal Zone N19/20 (e.g.,
Gibson, 1983; Hazel, 1983; Snyder et al., 1983),
and is referred to Coastal Onlap Cycle TB3.5 of
Haq etal. (1987). As discussed by Ward (1984,
1989) this relative sea level rise resulted in the
flooding of a sizeable part of the Coastal Plain in
the Salisbury embayment. The overlying
Rushmere and Morgarts Beach Members
represent the transgressive and highstand
systems tracts, respectively, of a depositional
sequence. Rushmere and Morgarts Beach
sediments were deposited during the most
extensive flooding of the Coastal Plain during the
Pliocene (Ward, 1984; Dowsett and Cronin, 1990).
These members (equivalent to most of Zone 2, or
the Turritella alticostata zone of the Yorktown
Formation) are correlated with the upper part of
Planktonic Foraminiferal Zone N19/20, and
Coastal Onlap Cycle TB3.6 (Dowsett and Cronin,
1990). The cross-bedded shell hashes of the
Moore House Member are indicative of highstand
deposits. Because of its disconformable
relationship to the underlying Morgarts Beach
Member, and the age constraints placed on the
stratigraphic position of the unit by the overlying
Chowan River Formation, the Moore House
Member is placed in the overlying TB3.7 Cycle.
The Moore House Member is known only from a
small area in the southeastern Virginian part of the
Salisbury embayment, and no deposits
representing Cycle TB3.7 are found in North
Carolina. The lithologies of the Edenhouse and
Colerain Beach Members of the Chowan River

SPECIAL PUBUCATION NO. 36

Formation are referred to transgressive and
highstand systems tracts, respectively, of a single
depositional sequence. A thin limonitic zone
separating the two members at some localities
(Hoffman and Ward, 1989) appears to represent
the condensed interval. The Chowan River
Formation is assigned to Planktonic Foraminiferal
Zone N21, and to Coastal Onlap Cycle TB3.8.
The TB3.8 Cycle is poorly represented in the
middle Atlantic Coastal Plain, being restricted to
a small area of the northeastern Albemarle
embayment. Sediments of the James City
Formation, characterized in the type area by the
development of Crepidula biostromes, appear to
represent highstand deposits (see DuBar et al.
1974; Miller and DuBar, 1988). However, there is
not sufficient data to determine the extent of
development of systems tracts within this
formation at present. The James City Formation
is considered to be early Pleistocene in age (see
Riggs and Belknap, 1988), and is assigned to
Planktonic Foraminiferal Zone N22 and Coastal
Onlap Cycle TB3.9. The James City Formation
and associated lower Pleistocene deposits to the
south record the most extensive relative sea level
rise since that responsible for deposition of the
Rushmere and Morgarts Beach Members of the
Yorktown Formation.

Analysis of lithostratigraphic and
biostratigraphic data for the Pliocene and lower
Pleistocene of Virginia and North Carolina
provides the basis for the development of the
sequence stratigraphic model depicted in Figure
2.

Pliocene and Pleistocene marine deposits
encountered in southwestern Florida consist of
siliciclastic, mixed siliciclastic-carbonate, and
carbonate lithologies whose lateral and temporal
relationships are obscured by thinness and
discontinuous distribution of units, limited
exposures, and rapid faces changes (see DuBar,
1974; Missimer, this volume). Many of these
deposits contain abundant invertebrate and some
vertebrate fossils, but calcareous micro- and
nannofossils are, for the main, absent in outcrop
and presumably destroyed by leaching and
recrystallization. Vertebrates, mollusks and
isotopic techniques allow gross correlation of

these sediments on a regional and, to some
extent, an intercontinental scale (Lyons, 1991).
However, correlation of individual lithologies
within these units is often beyond the resolution of
applicable chronostratigraphic zonations.

STARTIGRAPHIC INTERPRETATION

For the purpose of this study, the following
Pliocene-lower Pleistocene units and ages are
recognized in southern Florida:

Tamiami Formation Several lithologies, some
bearing formal lithostratigraphic names, are
loosely included in the Tamiami Formation in
southwestern Florida (see Missimer, 1990; this
volume). The Tamiami Limestone was named by
Mansfield (1939) for hard, light gray to white,
sandy, moldic calcarenite exposed in Collier and
Monroe Counties. Subsequent workers
incorporated additional lithologies in their
conception of the Tamiami, which prompted
Hunter (1968) to redefine the unit both in a litho-
and biostratigraphic sense (Figure 3). Hunter
recognized five formal members in the Tamiami
Formation. The basal Bayshore Clay was
proposed for white to light tan, sandy and pebbly
phosphatic clay in the Port Charlotte, Charlotte
County area. The overlying Murdock Station
Member, also from the Port Charlotte region, was
proposed for a thin unit consisting of lower
fossiliferous, pebbly, phosphatic clay and sand,
and upper locally indurated, phosphatic, medium-
to coarse-grained sand. The original Tamiami
Limestone of Mansfield was renamed the
Ochopee Limestone Member, and was considered
to be a lateral equivalent of the Buckingham
Limestone Member. This latter unit was named
by Mansfield (1939) in northeastern Lee County

Figure 3. Comparison of the Tamiami Formation of Hunter (1968) with the nomenclature proposed herein.

I

SPECIAL PUBUCATION NO. 36

for soft, light gray to white weathering buff,
slightly sandy and phosphatic calcilutite. The
Pinecrest Sand, first recognized by Mansfield
(1931) in the vicinity of Pinecrest, Monroe County,
and later recognized at quarries in Sarasota
County, is a highly fossiliferous quartz arenite that
Hunter (1968) also considered as a lateral
equivalent of the Buckingham and Ochopee
Limestone Members.

Scott (1988) restricted the Tamiami Formation
by incorporating the Bayshore Clay and Murdock
Station Member into the underlying Peace River
Formation of the Hawthorn Group. Missimer
(1990; this volume) recognized nine lithofacies
within the Tamiami Formation, and presented a
discussion of their age relationships. According
the Missimer (1990, figure 2) the Buckingham
Limestone and a tan clay and sand facies are the
basal lithologies of the Tamiami Formation. The
Ochopee Limestone, locally overlain by the Bonita
Springs Marl, a Hyotissa facies, and a sand facies
were considered to overlie these basal facies. In
turn, these lithologies were shown to be overlain
disconformably by an unnamed limestone facies
that grades laterally into the Golden Gate Reef
facies. The Pinecrest Sand was regarded as
being discontinuous in distribution and
disconformably overlying other faces of the
Tamiami Formation.

For the purposes of this study, as based on
observations of exposures in Sarasota, Charlotte
and Lee Counties and on published data, the
Tamiami Formation includes the Murdock Station
Member and equivalents at its base and ranges
upward through the Pinecrest Sand and
equivalents. The Tamiami Formation is here
divided into lower and upper parts. The lower
Tamiami Formation encompasses the Murdock
Station Member and the Ochopee and
Buckingham Limestones of Hunter (1968), and the
tan clay and sand, sand, Hyotissa, and Bonita
Springs faces of Missimer (1990; this volume).
The upper Tamiami is conformable on the lower
Tamiami, and includes two stratigraphic intervals
separated by a disconformity. The unnamed
limestone and Golden Gate faces of Missimer
(1990; this volume) and the lower part of the
Pinecrest Sand of Hunter (1968) are included in
what we informally term the lower Pinecrest beds.
The upper part of Hunter's Pinecrest Sand, here
termed the upper Pinecrest beds, is discon-
formable on the lower Pinecrest beds, and

appears to correlate with much of the Pinecrest
Sand of Missimer (1990; this volume).

This stratigraphic interpretation is best
exemplified by the sections exposed in the APAC
and Quality Aggregates pits in Sarasota County
(Figure 4). At the base of these pits cay of the
Peace River Formation of the Hawthom Group is
disconformably overlain by fossiliferous,
argillaceous, phosphatic, quartz arenite. This unit,
designated bed 11 in the APAC quarry by Petuch
(1982), contains the pectinids Chesapecten
septenarius and C. jeffersonius, other bivalves
such as the oyster Conradostrea sculpturata and
Mulinia congesta, large barnacles dominated by
Concavus tamiamiensis, the coral Septastrea
crassa, phosphatized bones, shark teeth, and
rip-up clasts from the underlying Peace River
Formation. Bed 10 of Petuch (1982), where
present, overlies bed 11 and is characterized by
closely packed shells of the bivalve Mercenaria
tridacnoides. Allmon (1990) indicated that in
some parts of the Sarasota pits bed 10 is
represented by channelling, reworked Mercenaria
shells, or a lag of blackened shell fragments,

Beds 11 and 10 are probably equivalent to
the Murdock Station Member. The lower
Murdock Station bed of densely packed pectinid
valves in a pebbly, phosphatic matrix of clay and
sand is similar to bed 11, and the upper bed of
broken oyster and echinoid shells in locally
indurated, phosphatic, medium- to coarse-grained
sand is similar to bed 10 in some areas. As is the
case with bed 11, the Murdock Station Member
contains no aragonitic fossils. Although the type
Murdock Station Member exposed in canals near
the Port Charlotte (formerly Murdock) railroad
station in Charlotte County is no longer available
for direct comparison with beds 11 and 10, we
conclude that the lithologic similarity and high
probability of correlation between these strata in
Port Charlotte and Sarasota argue strongly for
removal of the Murdock Station from the Peace
River and its return to the Tamiami Formation.

The Pinecrest Sand of Hunter (1968)
conformably overlies beds 11 and 10 at the pits in
Sarasota County. The Pinecrest is composed of
densely packed, poorly sorted, aragonitic and
calcitic shells of invertebrates (primarily mollusks)
in a clean, slightly phosphatic, quartz sand matrix.
Petuch (1982) described eight beds (Beds 9
through 2) within the Pinecrest in one of the

FLORIDA GEOLOGICAL SURVEY

APAC pits. However, these beds are not laterally
continuous throughout all of the pits on the APAC
property and, for the most part, are difficult to
recognize in the adjacent Quality Aggregate pits.
The Pinecrest has a diverse Invertebrate fauna
dominated by perhaps as many as 1200 mollusk
species (Allmon, 1990). As noted by Petuch
(1986) the name Pinecrest is preoccupied by a
Triassic unit in Utah. Petuch (1986) suggested
using the name Buckingham Formation for this
unit, but it is lithologically incompatible with the
typical Buckingham Limestone of the type area in
Lee County. As there does not appear to be an
available name for this lithology, we will use the
name Pinecrest beds in an informal sense for this
discussion.

Olsson and Petit (1964) concluded that a
disconformity separated the Pinecrest beds from
older lithologies of the Tamiami Formation, and
Petuch (1982), Ketcher (1990), and Missimer
(1990) Indicated that the Pinecrest beds are
disconformable on older Tamiami lithologies in
the APAC pits. However, we do not recognize
unconformities either between beds 11 and 10, or
between beds 10 and and the overlying Pinecrest
beds in the APAC and Quality Aggregates pits.
Detailed examination of the contact between beds
11 and 10 indicates that the visible changes are
entirely sedimentologic or biotic, and that the
hiatus between bed 10 and the Pinecrest
suggested by Allmon (1990) is the result of
sediment starvation and, thus, an omission
surface developed at a time of maximum flooding.
We consider the contact between beds 10 and
the Pinecrest beds to mark the boundary between
the lower and upper Tamiami Formation.

The upper Tamiami Formation as used here
includes the Pinecrest beds, as well as the
unnamed limestone facies and the Golden Gate
Reef facies of Missimer (1990). A disconformity
developed during subaerial exposure is present
within the Pinecrest, and is here used to subdivide
the Pinecrest into lower and upper parts. In the
APAC pit section described by Petuch (1982), this
disconformity is represented by the contact
between bed 4 (termed the black layer) and
overlying bed 3. Bed 4 is a dark, organic-rich
sand containing brackish, fresh water, and
terrestrial mollusks and terrestrial vertebrate
remains (Lyons, 1991). Bed 3 is characterized by
densely packed shells of the shallow marine
mytilid bivalve Perna conradiana in a matrix of

dean quartz sand. Bed 4 is not continuous
throughout the pits on the APAC property, and
has not been seen in the adjacent Quality
Aggregates pits. However, a well defined
disconformity is present in the Quality Aggregates
pits at the same stratigraphic horizon. Here,
bryozoan-annelid boundstone and sand and clay
laminae containing abundant plant rootlets and
worm burrows are overlain by densely packed,
Perna conradiana shells in a clean sand matrix.
Fresh water gastropods occur on the
unconformity. The surface of the underlying
laminated unit is highly irregular and bored.
Petuch's bed 4 at the APAC pits and the
laminated unit at the Quality Aggregates pits
represent a major shoaling event in the
depositional history of the Pinecrest beds (see
also Lyons, 1991), whereas the overlying Perna
conradiana bed represents a return to normal
marine conditions. Based on faunal and physical
characteristics, the lower Pinecrest is correlated
with the unnamed limestone and Golden Gate
facies, whereas the upper Pinecrest is probably a
correlative of most of the Pinecrest Sand as
depicted by Missimer (1990).

Age of the Tamiami Formation Three biostrati-
graphic zones were established by Hunter (1968)
for the Tamiami Formation. The basal Pecten
santamaria middlesexensis zone (= Chesapecten
middlesexensis) characterized the Bayshore Clay,
the Pecten jeffersonius zone (=Chesapecten
jeffersonius) was represented in the Murdock
Station Member, and the Pecten tamiamiensis
zone (='Chlamys' tamiamiensis) characterized
the Ochopee and Buckingham Umestones and
the Pinecrest Sand. On the basis of the
molluscan faunas of these zones, Hunter (1968)
correlated the Bayshore Clay with the St. Mary's
Formation in Virginia (= Eastover Formation of late
Miocene age). The Murdock Station Member was
correlated with Zone 1 of the Yorktown Formation
in Virginia (early Pliocene age). The units
characterized by the "C." tamiamiensis zone were
correlated with Zone 2 of the Yorktown Formation
(late Pliocene age). The Bayshore Clay, an upper
Miocene unit, is not considered further in this
discussion.

As concluded by Hunter (1968), the presence
of Chesapecten jeffersonius in the Murdock
Station Member and bed 11 should indicate
correlation with Zone 1 (=Sunken Meadow

SPECIAL PUBLICATION NO. 36

Member) of the Yorktown Formation (see Ward
and Blackwelder, 1975). However, the
co-occurrence of C. jeffersonius and C.
septenarius in bed 11 of the Tamiami Formation
poses a problem for correlation of the Florida
section with that of the middle Atlantic region, as
the latter species is restricted to Zone 2 of the
Yorktown Formation (Ward and Blackwelder,
1975). In our opinion, the first occurrence of C.
septenarius is of greater biostratigraphic
significance than the last appearance of its likely
ancestor, C. jeffersonius. In addition, the
presence in bed 11 of such typical Zone 2
Yorktown species as Mulinia congesta and other
mollusks listed by Ward (1990) strongly supports
correlation of bed 11 and, by inference, the
Murdock Station Member with Zone 2 of the
Yorktown Formation. Based on the barnacle and
pectinid fauna, bed 11 in Sarasota County
appears to correlate to Missimer's (1990) sand
facies and the Buckingham Limestone of Lee and
Charlotte Counties. The fauna and stratigraphic
relationships of bed 11 are further discussed in
papers by Ketcher, Waldrop and Wilson, and
Ward in this volume.

Planktonic foraminifera described by Akers
(1974) from the the Pinecrest beds (broad sense)
in the APAC pit are of Pliocene age (zone
N19/20), and the diverse mammalian fauna from
bed 4 of the APAC pit is of late Blancan (late
Pliocene) age (Jones 1990). However, overall
correlation of the Tamiami Formation with the
Neogene section of the Florida Panhandle and
that of the middle Atlantic Coastal Plain is based
on molluscan assemblages. Several molluscan
species in the Buckingham and Ochopee
Limestones, the quartz arenite facies of Lee and
Charlotte Counties, and the lower Pinecrest beds
indicate correlation with Zone 2 of the Yorktown
(see Hunter, 1968; DuBar, 1974; Jones, 1990;
Ward, 1990; Lyons, 1991).

The fauna of the upper Pinecrest beds in the
APAC and Quality Aggregates pits is not
specifically age diagnostic. Discussions by
Jones (1990) on the ostracode fauna and by
Ward (1990) and Lyons (1991) on the molluscan
fauna suggest that the upper Pinecrest is late
Pliocene in age. Ward (1990) tentatively
correlated the upper part of the Pinecrest beds
(broad sense) with the Chowan River Formation
of the middle Atlantic Coastal Plain. Lyons (1991)
noted that the upper Pinecrest (i.e., beds 3 and 2

and their presumed equivalents in Sarasota
County) contained a mix of Tamiami and
Caloosahatchee mollusks. Based on
superposition and sequence stratigraphy, we
would correlate the upper Pinecrest beds with the
uppermost Yorktown Formation (see below).

Caloosahatchee Formation The
Caloosahatchee Formation disconformably
overlies the Tamiami Formation (DuBar, 1958).
DuBar (1958, 1962) recognized three members in
the type area of the formation along the
Caloosahatchee River (in ascending order, the Ft.
Denaud, Bee Branch and Ayers Landing
Members), and six informal members in the Shell
Creek area of Charlotte County (beds A-F). The
Caloosahatchee is difficult to distinguish
lithologically from some facies in the underlying
Tamiami Formation. In practice, the
Caloosahatchee is recognized on its contained
fauna, and is, essentially, a biostratigraphic unit.

A well-defined disconformity is present
between Petuch's (1982) beds 2 and 1 in the
APAC pits. In the western part of the APAC
property, Bed 2 is a clean sand containing
abundant shells of the oyster Hyotissa haitensis
and a modest gastropod fauna. The upper part
of this unit is lithified, forming a prominent
limestone ledge. Disconformably overlying the
limestone is presumed nonmarine sand containing
abundant woody debris and tree stumps in life
position (Lyons, 1991). These organic-rich sands
are, in turn, overlain by fossiliferous sands
containing shallow marine to fresh water mollusks
and plant debris. The disconformity is not well
documented at the Quality Aggregates pits, and
according to Lyons (1991) the fossiliferous,
marine quartz sand immediately overlying the bed
equated with bed 2 at APAC may represent a part
of the section that is missing between the
limestone ledge (bed 2) and the overlying
wood-bearing sand at APAC. This disconformity
is taken as the contact between the Tamiami
Formation and the overlying Caloosahatchee
Formation. Petuch (1982) tentatively assigned
bed 1 to the Caloosahatchee Formation. Further
studies, as discussed by Jones (1990), Waldrop
and Wilson (1990), and Lyons (1991), indicate that
bed 1 and fossiliferous horizons equated with
Petuch's bed 0 at the APAC and Quality
Aggregates pits contain a Caloosahatchee fauna.

FLORIDA GEOLOGICAL SURVEY

Age of the Caloosahatchee Formation Based
on vertebrates and invertebrates (see DuBar,
1974) and on radiometric dates (see Blackwelder,
1981) the entire Caloosahatchee Formation was
correlated with the Waccamaw and James City
Formations of the Albemarle and Charleston
embayments and considered to be of early
Pleistocene (Calabrian) age. However, Ward and
Blackwelder (1987) correlated the lower
Caloosahatchee Formation (Ft. Denaud Member)
with the uppermost Pliocene Chowan River
Formation (Planktonic Foraminiferal Zone N21)
and, most recently, Lyons (1991) concluded that
all of the Caloosahatchee Formation was late
Pliocene in age. We follow Lyons in regarding the
Caloosahatchee as an upper Pliocene unit, and
correlate it with the Chowan River Formation of
the middle Atlantic region.

Bermont Formation DuBar (1974) informally
proposed the name Bermont Formation for the
uppermost gray, sandy, shell marl (formerly his
Unit F of the Caloosahatchee) disconformably
overlying the Caloosahatchee Formation along
Shell Creek in the Bermont quadrangle, Charlotte
County. The Bermont Formation also is
equivalent to Unit A of Olsson and Petit (1964).
As noted by DuBar (1974), the Bermont
Formation cannot be distinguished lithologically
from the Caloosahatchee Formation and is based
on faunal differences.

Age of the Bermont Formation We follow
Lyons (1991) who concluded that the Bermont
Formation, previously assigned to the middle
Pleistocene, is, in fact, early Pleistocene in age.

SEQUENCE STRATIGRAPHY

The Tamiami and Caloosahatchee Forma-
tions are here regarded as upper Pliocene units.
To date, no Pliocene deposits older than latest
Zanclean (calcareous nannofossil zone CN11)
have been recognized in southwestern Florida
either in outcrop or in the subsurface (J. M.
Covington, written communication, 1991).
According to Scott (oral communication, 1991)
planktonic foraminifera from a core in eastern
Florida indicate the presence of earlier Pliocene
deposits in that region, and it is likely that strata
of similar age are present downdip in southern
Florida.

Three Pliocene depositional sequences can
be recognized in southwestern Florida (Figures 2,
4). The oldest, assigned to the TB3.6 Cycle,
encompasses the lower Tamiami Formation and
the overlying lower Pinecrest beds, and is
correlated with the Rushmere and Morgarts Beach
Members of the Yorktown Formation. The
Murdock Station Member and bed 11 in Sarasota
County represent the transgressive deposits of the
sequence. The condensed interval (CI) is placed
in bed 10, which in some parts of the APAC and
Quality Aggregates pits of Sarasota County is
represented by a blackened (phosphatized?) shell
lag (Figure 4). The overlying lower Pinecrest
beds are highstand deposits of the TB3.6 Cycle,
characterized by a series of thicker tempestites
(mixed shell beds) alternating with thinner lower
energy deposits (e.g., Vermicularia beds), and
culminating in the marginal marine to paludal
sediments in the "black layer" of bed 4. The sand
facies of the Tamiami Formation exposed in the
Lomax-King pit in Charlotte County also
preserves transgressive and highstand tracts of
the TB3.6 Cycle. About 1 meter of fine to
medium transgressive sand at the base of the pit
is overlain by a few centimeters of lime mud. The
mud, representing the condensed interval, has a
pitted upper surface (omission surface) and is
overlain by about 2.5 m of shelly sand containing
large barnacles. Transgressive deposits of the
sand facies are also exposed in the Handy-Phil pit
in Charlotte County. The presence of
Chesapecten septenarius, and the barnacles
Concavus tamiamiensis and C. glyptopoma
support correlation with the Murdock Station
Member and with bed 11 in Sarasota County.
Highstand deposits at this pit are thin, intensely
weathered and eroded, and largely obscured by
paleosol development.

The upper Pinecrest beds, most of which
represent late highstand deposits, are assigned to
the TB3.7 Cycle and correlated to the Moore
House Member of the Yorktown Formation. The
upper Pinecrest in Sarasota County illustrates a
typical shoaling upward highstand tract. As
indicated previously, the fauna of the upper
Pinecrest beds is indicative of late Pliocene age,
but correlation with the Moore House Member is
based solely on superpositional relationships. In
our earlier interpretation (Zullo and Harris, 1990),
we indicated that the assignment of the Pinecrest
to either the TB3.6 or TB3.7 Cycles was depen-
dent upon the location of regional unconformities

SPECIAL PUBUCATION NO. 36

Figure 4. Nomenclature and sequence stratigraphic interpretation of the section in the APAC and Quality
Aggregates pits, Sarasota County. Stratigraphic section and terminology proposed by Petuch (1982, 1986)
for part of the APAC pit included for reference.

NOMENCLATURE SEQUENCE
STRATIGRAPHIC
DESCRIPTION OF BEDS I IINTERPRETATION
IN PART OF APAC PIT PETUCH THIS
AFTER PETUCH (1982) (1982, STUDY
1986) TRACT
O

within the Tamiami Formation. Recognition of the
contact between beds 10 and 9 as the surface of
maximum flooding in the TB3.6 Cycle, and
identification of an unconformity between beds 4
and 3 requires the assignment of the upper
Pinecrest beds to the younger cycle. Reassign-
ment of the Caloosahatchee Formation to the
upper Pliocene places an upper constraint on the
position of the upper Pinecrest within the
sequence of upper Pliocene cycles. A tentative
sequence stratigraphic scheme for the major
faces of the Tamiami Formation is illustrated in
Figure 5.

The Caloosahatchee Formation is assigned
to the uppermost Pliocene TB3.8 Cycle and
correlated to the Chowan River Formation of the
middle Atlantic Coastal Plain. Our earlier
interpretation (Zullo and Harris, 1990) placed the
Caloosahatchee Formation in Coastal Onlap
Cycle TB3.9, but following Lyons (1991), who
presented considerable evidence supporting a
late Pliocene age for the unit, the Caloosahatchee
is correlated to the youngest Pliocene cycle.
Bed A, a conglomeratic limestone in the Shell
Creek area, and part of the basal Ft. Denaud
Member in the type area appear to represent
transgressive deposits, whereas the overlying
units, which include fine-grained siliciclastic and

SARASOTA COUNTY

mixed siliciclastic-carbonate sediments of shallow
marine and freshwater origin, suggest late
highstand deposits. The Ft. Denaud transgressive
systems tract is, thus, a correlative of the
Edenhouse Member of the Chowan Formation,
and the overlying high stand deposits Included in
the Bee Branch and Ayers Landing Members are
correlatives of the Colerain Beach Member of the
Chowan Formation.

The Bermont Formation, following Lyons
(1991) who argued for an early Pleistocene age
for this unit, is placed in Coastal Onlap Cycle
TB3.9 and correlated with the James City
Formation of the middle Atlantic Coastal Plain.
Based on present evidence, the Bermont cannot
be assigned to systems tracts.

CONCLUSIONS AND FUTURE STUDIES

The sequence stratigraphic model proposed
for the middle Atlantic Coastal Plain can be
applied to the southern Florida section with
limited confidence. The conclusions to be derived
from this preliminary study are that: (1) the major
eustatic sea level rise documented by Atlantic
Coastal Plain deposits of the TB3.6 Cycle (Zone
2 of the Yorktown Formation, Duplin Formation,
Raysor Formation) is represented by the majority

of the Tamiami Formation; and (2) the younger
Pliocene and lower Pleistocene cycles TB3.7
through TB3.9 are represented in southern Florida
as they are in the mid-Atlantic region. Questions
regarding the sequence stratigraphic position and
the lateral and temporal relationships of the
various facies assigned to the Tamiami Formation
cannot be answered without detailed study of
systems tracts in outcrop, and the integration of
biostratigraphic and subsurface data with these
outcrop studies. Development of a sequence
stratigraphic framework can be of considerable
value in resolving the complex spatial and
superpositional relationships exhibited by the
Pliocene and lower Pleistocene of southern
Florida.

ACKNOWLEDGMENTS

The field studies undertaken for this research
project could not have been accomplished with-
out the invaluable assistance and generosity of
Warren D. Allmon, University of South Florida,
Roger W. Portell, Florida Museum of Natural
History, and Thomas M. Scott, Florida Geological
Survey, who also provided us with important data
concerning litho- and biostratigraphic
relationships. This study was made possible by
a grant from the Center for Marine Sciences
Research, University of North Carolina at
Wilmington, of which this is contribution no. 39.

Petuch, E. J., 1982, Notes on the molluscan
paleontology of the Pinecrest beds at Sarasota,
Florida with the description of Pyruella, a
stratigraphically important new genus
(Gastropoda: Melongenidae): Proceedings of the
Academy of Natural Sciences of Philadelphia, v.
134, p. 12-30.

1987, Late Pliocene and early
Pleistocene Mollusca from the James City and
Chowan River Formations at the Lee Creek Mine,
in Ray, C. E., ed., Geology and paleontology of
the Lee Creek Mine, North Carolina, II:
Smithsonian Contributions to Paleobiology no. 61,
p. 113-283.

New mapping of southern Florida sediments
and sedimentary rocks has defined three broad
Pleistocene units, Unit C (late Pliocene/early
Pleistocene), Unit D (middle Pleistocene) and Unit
E (late Pleistocene). Uranium-series analyses of
corals from these units show that no samples
have experienced ideal closed-system conditions
with respect to U and its daughter products, but
approximate ages can be estimated. We
estimate, from the U-series data, that Unit C is
>400 ka, Unit D is -230-360 ka, and part of Unit
E is -144 ka. The results indicate that marine
deposition was extensive over much of southern
Florida during the middle Pleistocene.

INTRODUCTION

The southern part of the state of Florida is a
generally low-relief landscape that is characterized
by carbonate-rich bedrock units of late Cenozoic
age. The rocks range from Tertiary (Hawthorn
Group, Tamiami Formation, Buckingham
Limestone, Ochopee Limestone, and Pinecrest
Sand) to late Pleistocene (Anastasia Formation,
Key Largo Limestone, and Miami Oolite). The age
of the Caloosahatchee Marl is not well con-
strained and could be Pliocene or early
Pleistocene or may well overlap the boundary
between these two epochs.

The sediments in southern Florida are
mainly of marine origin, and their ages could give
considerable insight into late Cenozoic sea-level
history if they can be accurately dated. Marine
sediments that were deposited in warm,
low-latitude waters often contain fossil corals, and

these organisms are the most suitable materials
for 23U/2U and 2 'h/23U dating. Uranium-series
dating of corals is based on the observation that
corals take up U, but no Th into their skeletal
structures during growth, and generally behave as
closed systems with respect to U and its daughter
products after death. Thus, measurement of the
activity ratios of daughter to parent ("U/2'U and
23'Th/2U) can yield the age of the coral and
closely date the time of deposition.

There have been three previous attempts to
date corals from late Quaternary limestones in
southern Florida. Broecker and Thurber (1965)
and Osmond and others (1965) used uranium-
series methods to date the coral-bearing Key
Largo Limestone and ooids (which also take up U
during precipitation from sea water) from the
Miami Oolite. Some of their analyses were done
on samples that contained secondary calcite
present as cement or due to in situ recrystal-
lization of aragonite. However, of those samples
they analyzed that contained 95-100% aragonite,
they reported ages of 90 9 and 120 10 ka for
the Miami Oolite and a range of ages from 95
9 to 145 14 ka for the Key Largo Limestone. A
Siderastrea siderea sample from the quarry in the
Key Largo Limestone at Windley Key was also
analyzed by a number of laboratories as part of
the Uranium-Series Intercomparison Project
(Harmon and others, 1979). This sample con-
tained only 90% aragonite and gave a mean age
of 139 ka. The older apparent age could be due
to some U loss as a result of recrystallization of
aragonite. Even with the large uncertainties
associated with all of these previous results, the
data suggest that both units correlate with the
peak of the last interglacial complex, which is
generally thought to have occurred around

SPECIAL PUBUCATION NO. 36

120-130 ka, based on studies conducted on
Barbados, New Guinea, Haiti, and many other
localities (see summary in Muhs, 1992).

There has been, to our knowledge, only one
attempt to apply U-series methods to dating
corals from Florida formations that are older than
the last interglacial. Bender (1973) applied
hITh/~U,U/'U, and 'He/U methods to dating
corals from the Pinecrest and Caloosahatchee
Formations. All but one of his coral samples
show evidence of U loss, but iTh/24U and
WU/mU values of the other sample are at
equilibrium (within analytical uncertainty) and
indicate ages of >350 ka and >1.0 Ma,
respectively. His 'He/U values suggest ages of
-3-4 Ma for the Pinecrest Formation and -1.8
Ma for the Caloosahatchee Formation, which are
consistent with the stratigraphic relations, but the
evidence for U loss makes these ages tentative.

In this study, we collected corals from
various localities in southern Florida for U-series
dating in an attempt to shed new light on the
possible ages of some of these units. These

Almon (1990)

studies are being carried out in collaboration with
new mapping efforts in parts of the region, which
have been recently summarized by McCartan and
others (1991). All corals that we analyzed are
95-100% aragonite and analyses were done by
isotope-dilution alpha spectrometry, following the
laboratory methods outlined by Muhs (1992).

LATE CENOZOIC STRATIGRAPHY OF
SOUTHERN FLORIDA

The stratigraphy of late Cenozoic sediments
and sedimentary rocks in southem Florida is not
completely agreed upon and many of the pro-
blems that exist have been summarized elsewhere
in this volume. In Figure 1, we summarize the
classic late Cenozoic stratigraphy of southern
Florida as given by Allmon (1990), with correlative
units in the new framework modified from
McCartan and others (1991). Corals were
analyzed from Units C (Pliocene or early
Pleistocene) and D (middle Pleistocene) and from
the Bermont Formation as recognized in the field
by B. Blackwelder. Two samples from the 05
(late Pleistocene) and Q4 (middle Pleistocene;

terminology of Perkins, 1977) members of the Key
Largo Limestone, taken from a drill core from the
island of Key Largo were also analyzed. The
distribution of the units and sample localities are
given in Figure 2.

URANIUM-SERIES SYSTEMATICS

Several criteria are used to determine if
U-series ages of corals are reliable, or have
experienced closed-system conditions with regard
to U and its daughter products. The U
concentrations in fossil corals should be similar to
concentrations in living corals, which are usually
between about 2 and 4 ppm. The "Trh/23'Th
value should be high (>20), indicating no
"inherited" 2mTh. Finally, the 2lTh/3U values
should be consistent with the MU/'U values,
based on the assumption that sea water in the
past had the same uranium Isotopic composition
as sea water of the present (1.135 to 1.155, as
reported by Chen and others, 1986), and no Th
was present initially.

None of our samples meet all of the
closed-system criteria given above (Table 1).
Sample FL-15 has a U concentration that is much
higher than is typical for modern corals and has
therefore experienced significant secondary U
addition (Table 1). Uranium gain is the probable
reason for the young apparent age for a coral
from the Caloosahatchee Formation (Unit B) that
may be Pliocene or early Pleistocene. FL-14 and
FL-14B have Th ratios that are significantly lower
than 20 and therefore have some initial 2 Th
contamination. Contamination by inherited '"Th
can be corrected by analysis of the host
sediment, which is the likely contaminant, or by
assuming a probable value for the host sediment.
All samples analyzed lack concordance between
'U/'Uvalues and "'Th/maUvalues. Figure 3
shows the theoretical isotopic evolution of corals
that Incorporated U from sea water that have U
isotopic compositions of 1.135 and 1.155, based
on the range of values for modern sea water
reported by Chen and others (1986). Samples
that have experienced closed-system conditions
should plot on or between these lines, within
analytical error. It is apparent from the data
shown in Figure 3 that none of our samples meet
these criteria. The majority of samples have
mU/mU values that plot above the idealized
curves. Most samples (with the exception of
FL-15) have U concentrations that are within the

normal range for corals. We infer from the normal
U concentrations combined with the higher than
expected mU/mU values that the corals have
experienced 2mU additions, probably from
alpha-recoil processes. Ku and others (1990)
showed that m2U additions from alpha-recoil
processes do not greatly change the ages of
corals. Therefore, although the apparent ages are
minima, in some cases they may be close
minima. Four of our samples plot below the
idealized curves (i.e., have lower 'U/"U values
than would be expected for their "Th/mU ages)
and it is more difficult to explain the processes
that might have produced the lower values. One
possibility is that the U taken up by the corals
was in part derived from old U-bearing limestone
formations or phosphate deposits that have low
4mU/mU values and the U derived from such
sources was not well mixed with open ocean
water. Observations of lower than expected
mU/2U values in corals were also made on
Bermuda, a carbonate-dominated Island, by
Harmon and others (1983). Several samples
(FL-17A, FL-17C, FL-25, FL-26, and FL-28) have
2"Th/mU values that are too high for corals that
have experienced closed-system conditions.
Such a condition is referred to as excess, or
unsupported 2"Th,and is probably due to U loss
during the samples' histories. No ages can be
calculated for samples with unsupported 'Th.

URANIUM-SERIES AGE ESTIMATES

Pliocene/Early Pleistocene (Unit C) corals

Four samples were analyzed from Unit C,
which could be late Pliocene or early Pleistocene
(Table 1). Unit C includes what has been mapped
as the Caloosahatchee Marl by previous workers.
Two samples (FL-17A and FL-17C) have "'Th
excesses and no ages can be calculated. One
sample (FL-15) has a very young apparent age,
but the U concentration is too high, indicating
secondary U addition at some relatively recent
time in the sample's history. Only one sample
(FL-19) gave a reasonable age estimate of >400
ka. However, the 2U/mU value is too high for
its "3Th/mU age, indicating that the minimum
age estimate is even more of a "minimum."

Middle Pleistocene (Unit D) corals

A number of probable middle Pleistocene
corals from southern Florida were analyzed. One

SPECIAL PUBUCATION NO. 36

Figure 2. Geologic map of southern Florida and coral sample localities. The Key Largo Limestone
and Miami Oolite boundaries are generalized from Scott and others (1986); other units are
from McCartan and others (1991) and unpublished data of McCartan.

Isotopic evolution curves with solid lines showing theoretical change over time in
composition of corals that had initial 'U/mUt values of 1.135 and 1.155 and no initial "Th
(ocean water values from Chen and others, 1986). Values plotted are for coral data given
in Table 1; for clarity, only MU/2mU error bars are given ( 1 sigma).

Montastrea (FL-30) came from a drill core taken
from the Q4 unit of the Key Largo Limestone on
the island of Key Largo, which underlies the late
Quaternary Key Largo Limestone member Q5 that
has been previously dated at around 95 ka to 145
ka by Osmond and others (1965) and Broecker
and Thurber (1965). The Montastrea sample was
the only aragonitic coral recovered from the Q4
unit during drilling described by Harrison and
others (1984) and Harrison and Coniglio (1985).
Because the sample has experienced some
secondary addition of 24U (Fig. 3), the age
estimate of 361 +120/-61 ka is a minimum.

On the north end of Lake Okeechobee and
elsewhere (Fig. 2), probable middle Pleistocene
(Unit D) sediments are not exposed at the
surface, but are buried beneath thin deposits of
late Pleistocene (Unit E) age. A Solenastrea from
unit D (FL-16) collected near the town of

Okeechobee gives an apparent age of 330 21
ka. With the exception of a slightly higher
234U/3U ratio, the age estimate appears to be
reasonable. Two Solenastrea samples collected
from what was described as the Bermont Forma-
tion by Blake Blackwelder near the southern end
of Lake Okeechobee were also analyzed (Table
1). One (FL-28) has excess '3Th, but the other
(FL-27) gives an apparent age of 266 40 ka. As
was the case with FL-16, the 2U/mU ratio is too
high for the apparent age, so the age is a
minimum estimate. We are not certain whether
the host sediment, described as "Bermont,"
correlates with Unit D found on the north end of
Lake Okeechobee, but the U-series data permit
such a correlation. A similar age was derived for
FL-18, collected from Punta Gorda farther to the
west (Fig. 2). At Punta Gorda, the coral has a
lower than expected mU/mU ratio for its
2"Th/mU age, so we are uncertain about a

Figure 3.

FLORIDA GEOLOGICAL SURVEY

possible correlation with Unit D sediments found
in the Lake Okeechobee area. Two Septastrea
samples (FL-14 and FL-14B) collected from a pit
east of Orlando in sediments that we have
mapped as Unit D (Fig. 2) both have '"Th/"'Th
ratios significantly lower than 20, which indicates
that they contain some inherited 2"Th. Assuming
that the host sediment is the contaminant and that
it has a 0DTh/uaThvalue of 1.5, which is common
for calcareous sediments (Szabo and others,
1982), we derive corrected ages of -236 ka
(FL-14) and -291 ka (FL-14B) using the
correction scheme of Szabo and others (1982).
Both corals have uranium isotopic compositions
that are too low for their apparent ages, so we
regard the corrected age estimates as tentative.

Collectively, our data suggest that there may
have been extensive deposition of marine sedi-
ments, represented by Unit D, over southern
Florida during the middle Pleistocene. Marine
transgressions during the middle Pleistocene
covered an area of southern Florida from the
Keys to at least as far north as Orlando. The
U-series age estimates suggest that much of the
marine sedimentation took place between -200
ka and -400 ka. On the island of Barbados, at
least five uplifted coral reef tracts that represent
discrete high stands of sea have been dated by
2"Th/24U, 4He/U and electron-spin resonance
methods and occur in this time interval (Bender
and others, 1979; Radtke and others, 1988). At
least some of the high stands are not recorded in
some parts of Florida, however. In the drill core
samples taken from Key Largo, the minimum age
estimate of 300 ka for FL-30 and -144 ka age for
the overlying 05 member (FL-29: see later
discussion) suggest that coral growth in part of
the Florida Keys, corresponding to the 180-220 ka
high stand of sea recorded on Barbados,
Bermuda, and the Bahamas (Bender and others,
1979; Harmon and others, 1983; Muhs and
others, 1987; Foos and Muhs, 1991) is missing.
Harmon and others (1983) report that on
tectonically stable Bermuda, the 180-220 ka high
stand of sea is recorded as patch reefs at +2 m.
Uranium-series dating of ooids and peloids from
tectonically stable New Providence and San
Salvador Islands in the Bahamas indicates that
sea level could have been no lower than about -5
m at 190-225 ka (Muhs and others, 1987; Foos
and. Muhs, 1991). Thus, evidence from both
Bermuda and the Bahamas indicates that sea
level was high enough that this stand of sea

should have been recorded as reef growth on the
Florida Keys. However, deposits between -150
ka and -350 ka have not been identified in the
Florida Keys and appear to be missing beneath
Key Largo.

Late Pleistocene (Unit E) coral

One Montastrea sample (FL-29) from the
upper member of the Key Largo Limestone (the
05 unit of Perkins, 1977) collected from a drill
core taken on the island of Key Largo gives an
age of 144 8 ka, which is in agreement with the
oldest ages for the unit reported by Broecker and
Thurber (1965), Osmond and others (1965), and
Harmon and others (1979). The Key Largo
Limestone has commonly been correlated with
the peak of the last interglacial complex, but we
note that the apparent age of -144 ka is older
than is typical for corals that have been correlated
with the high sea stand. Some investigators have
suggested that there were two distinct high
stands of sea in the time interval from about 120
ka to 140 ka, rather than one broad high stand of
sea centering on -125 ka (Steams, 1976;
Chappell and Veeh, 1978; Aharon and others,
1980; Moore, 1982). It is possible that the Key
Largo Limestone represents the earlier high stand
of sea and the Miami Oolite represents the later
high stand of sea; alternatively, the range of ages
may suggest that there was a single, but long
high stand of sea. The U-series ages reported by
Broecker and Thurber (1965) and Osmond and
others (1965) as well as our data permit either
interpretation. The problem might be resolved by
high-precision U-series analyses of both the Key
Largo Limestone and Miami Oolite by mass spec-
trometric methods.

SUMMARY AND CONCLUSIONS

Our oldest unit, Unit C, which includes the
classic Caloosahatchee Formation, is greater than
400 ka and is consistent with previous age
estimates by the 'He/U method which suggest
that it is about 1.8 Ma. Unit D is of middle
S-Pleistocene age and contains corals that range in
age from -230 ka to -360 ka. More than one
high stand of sea may be represented by Unit D
and at least five terraces on Barbados, ranging in
age from -200 ka to -400 ka, may correlate with
parts of Unit D. Reefs that are 180-220 ka,
however, are missing from the stratigraphic
sequence on the Florida Keys based on U-series

SPECIAL PUBUCATION NO. 36

dating we report here. It is also possible that
reefs of this age exist on the Keys, but have not
yet been discovered. The youngest unit we have
studied (part of Unit E) is the younger member of
the Key Largo Limestone, and with an age of
-144 ka is in agreement with previous estimates.

On the basis of the isotopic systematics, we
conclude that corals from southern Florida marine
deposits are not ideal materials for U-series
dating. However, our data suggest that there
may have been extensive marine deposition in
southern Florida during the middle Pleistocene.
Our age estimates invite further testing by other
dating methods, particularly aminostratigraphy,
Sr-isotope stratigraphy, and biostratigraphy.

ACKNOWLEDGMENTS

This study was supported in part by the
Global Change and Climate History and National
Geologic Mapping Programs of the U.S.
Geological Survey. We thank Blake Blackwelder
(U.S. Geological Survey) for supplying some of
the corals and Steve Calms (National Museum of
Natural History) for identifying the coral species.
Lynn Wingard and Ken Ludwig of the U.S.
Geological Survey helped clarify the stratigraphy
on the basis of unpublished paleontologic and
Sr-isotope data. Barbara Udz and Meyer Rubin
(both of the U.S. Geological Survey) read an
earlier version of the paper and made helpful
comments for its improvement.

Foos, A.M., and Muhs, D.R., 1991, Uranium-series
age of an oolitic-peloidal eolianite, San Salvador
Island, Bahamas: New evidence for a high stand
of sea at 200-225 ka: Geological Society of
America Abstracts with Programs, v. 23, no. 1, p.
31.

Muhs, D.R., 1992, The last interglacial/glacial
transition in North America: Evidence from
uranium-series dating of coastal deposits, in
Clark, P., and Lea, P., eds., The last
interglacial/glacial transition in North America:
Geological Society of America Special Paper 27,
in press.

The shallow-marine strata of southern Florida
contain a richly fossiliferous record of Pliocene
and Pleistocene organisms, reflecting patterns of
physical and biological change during the past
several million years. Occasionally nonmarine
units containing fossils of freshwater and
terrestrial organisms are interbedded with the
marine deposits, providing the opportunity to link
the biotic records in both realms directly. To
construct a precise temporal framework at such
important sites, several stratigraphic approaches,
including invertebrate and vertebrate biostra-
tigraphies, strontium isotope stratigraphy, and
magnetostratigraphy were integrated at each
locality. The results at two such sites, the Leisey
Shell Pit near Tampa Bay and the APAC Shell Pit
in Sarasota, indicate that age constraints within
the range of 0.5 m.y. were attainable. This
integrative approach represents a powerful
technique for resolving geochronologic questions
in Florida, particularly during portions of the
Neogene most amenable to strontium isotope
stratigraphy.

INTRODUCTION

The gradual development of the Panamanian
Land Bridge and connection of the North and
South American continents during the Pliocene
precipitated a series of dramatic changes around
the Caribbean Basin and beyond. Associated
with the closure of the Central American Seaway
are the:

While it is generally recognized that these major
features are broadly related to one another, it is
not evident precisely how they are linked, either
chronologically or mechanistically.

Paleontologists have long recognized the late
Neogene as a time of major extinction pulses as
well as climatic and biogeographic change.
Whether working in terrestrial or marine
paleoenvironments in North America, they have
often cited climatic fluctuations as causal factors
behind late Neogene faunal changes (e.g., Webb,
1974; Stanley, 1986). The key to understanding
these relationships is to investigate strata of the
appropriate age which may contain information
useful for addressing these important questions.
Fortunately Florida contains a robust, fossiliferous,
late Neogene stratigraphic record with a great
deal of potential for addressing the paleobiotic
and paleoenvironmental issues cited above.

Recognizing this potential, a group of
investigators at the University of Florida (UF)
became interested in combining expertise to
examine the record of late Neogene changes in
Florida. This multi-disciplinary approach has
focused upon the identification of key sites which
preserve interbedded marine and non-marine
faunas so that the chronology of terrestrial
change may be directly tied to corresponding
events in the marine realm. To date, two such
sites have been investigated in detail, both of
which are commercial sand and shell mines
producing extraordinary vertebrate and
invertebrate fossil remains. These Include the
Leisey Shell Pit on the southeastern edge of

SPECIAL PUBLICATION NO. 36

Tampa Bay and the APAC Shell Pit adjacent to
1-75 South in Sarasota County. The results of
both investigations have already been published
(Leisey-Webb et al., 1989; APAC-Jones et al.,
1991).

The purpose of this paper is to acquaint the
reader with the multi-disciplinary approach used
in these investigations, to briefly discuss the
various geochronologic methods employed, and
to summarize the results obtained to date at
Leisey and APAC. Many people have contributed
their special skills and knowledge to this project
and this paper merely highlights some of their
collective efforts and input. The members of the
working group responsible for the data presented
herein include faculty in the Florida Museum of
Natural History and the Department of Geology at
UF (S.D. Webb, B.J. MacFadden, D.S. Jones, P.A.
Mueller, and D.A. Hodell), as well as staff at the
museum (G.S. Morgan, R.C. Hulbert, Jr.) and the
U.S. Geological Survey (T.M. Cronin). All of these
individuals should properly be considered
co-authors of this paper, although any errors of
fact or interpretation are solely my own.
Numerous other UF staff members and students,
past and present, also assisted with field and
laboratory work.

METHODS: INTEGRATIVE
GEOCHRONOLOGY

A prerequisite to understanding late Neogene
biotic and environmental changes in Florida is the
development of a sound temporal framework
within which the degree and rate of change can
be measured. Though Florida's Plio-Pleistocene
stratigraphic record has been studied for over a
century, major controversies abound concerning
the age assignments of particular units and
faunas. Several factors have contributed to this
condition: 1) lithostratigraphic analysis is
hampered by the fact that biogenic components
(e.g., shells) often constitute the major
sedimentologic elements in some units with
nondescript mixtures of sand and silt forming the
remainder of the sediment; 2) biocorrelation using
age-diagnostic planktonic foraminifera or
calcareous nannoplankton is often impeded due
to the nearshore paleoenvironments which did not
favor the preservation of such open-marine forms;
3) limited exposures of short sections (e.g.,
quarries, stream banks, excavations, etc.) inhibit

physical correlation of beds within the region; 4)
the carbonate environments and semi-tropical
nature of the marine invertebrate faunas from
Florida often do not readily correlate with
contemporaneous units and faunas in the Gulf
and Atlantic Coastal Plains to the north; and 5)
absolute dating of these sequences has proven
difficult as most of the units of interest are beyond
the range of "C and not amenable to other
standard radiometric methods. Therefore, we
have approached the problem of developing a
chronologic framework for the Plio-Pleistocene of
southern Florida by integrating several methods
(including vertebrate and invertebrate
biochronology, paleomagnetism, and stronium
isotope stratigraphy) at key sections throughout
the state.

INVERTEBRATE BIOCHRONOLOGY

Most previous invertebrate biochronologic
investigations in the Plio-Pleistocene strata of
Florida have focused upon the abundant and well
preserved molluscan fossils. The earliest attempts
to date these rich faunas involved the use of
Lyellian percentages (fraction of fossil species
surviving to the Recent). Not realizing that the
tropical western Atlantic experienced unusually
heavy extinctions in the late Neogene (Stanley
and Campbell, 1981), these early age
assessments tended to over-estimate the age of
the faunas. Current efforts emphasize the
correlation of the molluscan faunas from Florida
with comparable faunas to the north whose ages
are presumably better constrained through the
use of microfossils or nannofossils (e.g., Stanley,
1986). Rather than relying on single index or key
taxa, this approach normally involves arguments
for temporal equivalence based upon high
percentages of shared taxa.

In addition to molluscs, ostracodes have been
used with increasing effectiveness in
biostratigraphic studies of this region (e.g.,
Cronin, 1990; Hazel, 1983). Ostracode
abundance and preservation in marginal marine
settings make them particularly valuable in the
Florida sequences, such as at the APAC site
(Jones et al., 1991).

VERTEBRATE BIOCHRONOLOGY

Rich vertebrate faunas are known from
Pliocene and Pleistocene deposits throughout

FLORIDA GEOLOGICAL SURVEY

Florida. Those faunas which contain mammalian
components can often be placed quite precisely
within the accepted scheme of land mammal ages
adopted for North American Quaternary
sequences (Lundellus et al., 1987). These land
mammal ages have been tied to the geomagnetic
polarity time scale which further enhances their
utility in stratigraphy and geochronology (Figure
1). The three land mammal ages that cover most
of the successive changes in the North American
Plio-Pleistocene mammalian fauna include the
Blancan, the Irvingtonian, and the Rancholabrean,
each based on the first appearance of certain
immigrant and endemic taxa (Lundelius et al.,
1987). Preceding these is the Hemphillian Land
Mammal Age which spans the Late Miocene and
earliest Pliocene, with the Hemphillian-Blancan
boundary falling within the Gilbert Chron (Figure
1), between about 4.0 and 4.4 Ma. The
Blancan-lrvingtonian boundary is better defined,
occurring at the Olduvai subchron (about 1.88
Ma), approximating the Pliocene-Pleistocene
boundary. The Irvingtonian Land Mammal Age
spans most of the Pleistocene, ending with the
appearance of Bison in North America around 0.3
Ma. Nevertheless, the beginning of the
Rancholabrean is poorly dated, with estimates
ranging from 0.2 to 0.55 Ma (Lundelius et al.,
1987). Widely recognized subdivisions for each of
the land mammal ages (usually based upon first
or last appearance datums or stage of evolution
estimates of rapidly evolving lineages such as
horses or rodents see summary in Lundelius et
al., 1987) often permit even more refined
biostratigraphic assessments (i.e., "sub-ages", or
at least temporally restrictive designations such as
early or late).

Vertebrate faunas of middle to Late Pliocene
age (Blancan Land Mammal Age) are fairly rare in
the rich Neogene fossil record of Florida, while
faunas representing the Hemphillian and the
Irvingtonian are relatively common (Morgan and
Ridgway, 1987). Despite this uneven distribution,
at least nine significant Blancan vertebrate faunas
are recognized from Florida, with four of these
known in considerable detail (Morgan and
Ridgway, 1987). The majority of the Blancan sites
are located in southwestern Florida. Hemphillian
sites are scattered throughout Florida, but the
best known faunas are concentrated in northern
Florida and in the Bone Valley region.
Irvingtonian faunas are well represented
throughout the state, particularly in southwestern

Florida, and Rancholabrean sites are also widely
distributed.

MAGNETOSTRATIGRAPHY

The character of the magnetic polarity time
scale is well constrained throughout the Pliocene
and Pleistocene, making it a valuable stratigraphic
tool for correlating blochronologic units of either
marine or terrestrial origin (e.g., Undsay et al.,
1987). However, two factors have hindered the
wider application of magnetostratigraphic studies
in southern Florida: 1) the comparatively short
stratigraphic sections exposed at most sites rarely
permit the establishment of a reversal chronology;
and 2) the major component of many deposits is
biogenic carbonate (shells) which is not
conducive to paleomagnetic analysis.

The latter complication was recently overcome
by taking an innovative approach to sample
acquisition (Webb et al., 1989). At the Leisey
Shell Pit, oriented samples of large molluscs
having fine-grained sediment in-fillings were
collected throughout the sequence. The oriented
in-fillings were then impregnated with
non-magnetic hardeners and drilled to produce
measurable magnetic samples. The problem of
short sections still exists, however, and it is
unlikely that long reversal chronologies will be
attainable from the surficial outcrops of southern
Florida. In this case we use the magnetic polarity
in conjunction with other available chronologic
data to place age constraints on the sequence
under investigation (see Webb et al., 1989).

STRONTIUM ISOTOPE STRATIGRAPHY

The decade of the 1980s witnessed the
evolution of strontium isotope ("Sr/"Sr)
stratigraphy as a major geochronologic technique.
Studies of marine carbonates have demonstrated
significant and regular variations in the "Sr/"Sr
ratio of seawater throughout geologic time.
These studies have also shown that during certain
intervals of rapid change in Sr isotopic ratios with
respect to time, the "Sr/"Sr ratio can be used for
rather precise relative and absolute age
determination of marine carbonates (e.g., Burke et
al., 1982; Palmer and Elderfield, 1985; DePaolo
and Ingram, 1985; DePaolo, 1986; Hess et al.,
1986; Elderfield, 1986; McKenzie et al., 1988;
Veizer, 1989; Capo and DePaolo, 1990; Hodell et
al., 1991).

SPECIAL PUBUCATION NO. 36

RA F RVINGTONIAN BLANCAN I HEM
Brunhes Matuyama Gauss Gilbert

-I

*
Smm
0 '

'a
L A
IAluma 9A

Leisey

aE
a
=

APAC

~IE III.- ~I~ -I

0 588
0 590

Lelsey 1A '"

APAC a -
Lelsey 3

0.70915'

0.70910-

0.70905

ma

I IWL .P' U

Age (millions of years)

Age assessments for "bone bed" samples from Leisey 1A and 3A and APAC shell pits based
upon integrative stratigraphic approach. "Sr/"Sr ratios for bone bed samples are plotted
against known variation in seawater Sr isotopic composition for the Plio-Pleistocene front
DSDP sites 590 and 588 (Capo and DePaolo, 1990; Hodell et al., 1991). North American
land mammal ages, correlated to the magnetic polarity time scale, are plotted across the
top and help constrain the age estimates of the vertebrate faunas at these two sites (thick
black lines).

RESULTS: INTEGRATED GEOCHRONOLOGY
AT LEISEY AND APAC SHELL PITS

LEISEY SHELL PIT

The Leisey Shell Pit is situated on Little
Cockroach Bay on the southeastern edge of
Tampa Bay in the western half of Section 15,
T32S, R18E, Ruskin Quadrangle, Hillsborough
County. Mining operations exposed a major bone
bed there in 1983 and paleontological excavations
have subsequently revealed the largest Early
Pleistocene (Irvingtonian) vertebrate fauna in
North America, consisting of over 30,000
catalogued specimens representing over 100
species. In 1986 a similar but less extensive bone
bed was discovered about 0.5 km to the north, in
another area of the mining operations. Both the
former and latter bone beds (referred to as Leisey
1A and 3A, respectively) accumulated as
organic-rich deposits about 5-30 cm thick,
sandwiched. between massive marine shell beds
(Figure 2). Details of the physical stratigraphy,

Recent refinements of selected intervals of the
global seawater Sr isotope curve (e.g., Figure 1)
have revealed that particular segments are
amenable to high resolution stratigraphy. Since
much of the Neogene is characterized by a rapid
increase in the Sr isotopic ratios (DePaolo, 1986;
Hodell et al., 1991), the marine, carbonate-rich
strata of southern Florida is a likely target for Sr
stratigraphic studies. To date, two such
investigations have been undertaken in the
Pliocene-Pleistocene of Florida: 1) the Leisey
Shell Pit (Webb et al., 1989); and 2) the APAC
Shell Pit (Jones et al., 1991). In each case,
biogenic marine carbonates were collected from
throughout the section. We used unaltered,
aragonitic shells of the bivalve Chione sp. which
were cleaned and prepared according to standard
techniques (McKenzie et al., 1988; Webb et al.,
1989). Strontium isotopic ratios were measured
on a triple collector, VG Isomass 354 mass
spectrometer located in the Department of
Geology, UF.

paleoecology and taphonomy, as well as an
elaboration of the synopsis provided below may
be found in Webb et al. (1989) and Hulbert and
Morgan (1989).

All of the strata between the Hawthorn Group
and the top of the two main bone beds were
informally referred to the Bermont Formation,
primarily on the basis of the molluscan fauna.
This was done with some reservation, realizing the
problems associated with correlating shell beds in
southern Florida. Three typical Bermont mollusks
have been recovered from Leisey: the bivalve
Miltha carmenae and the gastropods Fasciolaria
okeechobiensis and Strombus mayacensis. The
molluscan fauna consists of about 200 species,
predominantly marine and evenly divided between
bivalves and gastropods. The nonmarine
components are principally restricted to the bone
beds. Of the remaining 190 species, 165 are
extant and 25 (12% of the total fauna) are extinct.
The Bermont has from 10-20% extinct species
and occupies a position between the older
Caloosahatchee Formation and the younger Fort
Thompson Formation (Hoerle, 1970; DuBar,
1974). The molluscan fauna of the former has
from 50-65% extinct species whereas the latter
typically has fewer than 5% (DuBar, 1958; Stanley,
1986). The few published dates and invertebrate
biochronologic data for the Bermont suggest an
age of about 0.5 Ma for this unit; however, other
lines of evidence discussed below indicate an
Early Pleistocene age for the Bermont Formation
at the Leisey Shell Pit.

Mammalian fossils from this site indicate an
early but not earliest Irvingtonian age. This
assignment is based especially on first
appearance data of immigrant taxa from both the
Old World and the Neotropics as well as
overlapping range zones of indicator species and
absences of presumably extinct taxa. Three
diagnostic early Irvingtonian mammals found at
Leisey include Mammuthus, Smilodon, and Lepus.
Other genera which first appear in North America
during the Irvingtonian and are found at Leisey
are the giant beaver (Castoroides) and the otter
(Lutra). Among post-Blancan, immigrant taxa
from South America recovered here are the
capybara (Hydrochoerus), two kinds of sloths
(Nothrotheriops and Eremotherium), and a new
glyptodont. Most Irvingtonian faunas are further
characterized by the absence of such typical
Blancan genera as Nannippus, Equus

(Dolichohippus), Borophagus, Hypolagus, and
Procastoroides, none of which is found at Leisey.

Because the Irvingtonian encompasses most
of the Pleistocene (ca. 1.9 to 0.3 Ma), more
precise subdivisions are desirable. In their review
of Irvingtonian biochronology, Lundelius et al.
(1987) recognized three subages, Sappan,
Cudahyan, and Sheridanian. The mammalian
fauna from Leisey was assigned to the later part
of the Sappan subage (i.e., late early
Irvingtonian), with the mammoth (M. meridionalis)
and the sabercat (Smilodon gracilis) figuring
prominently in this assignment. From
radiometrically dated late Sappan sites in the
western U.S., an age of about 1.5 to 1.2 Ma may
be indirectly inferred for the Leisey vertebrate
fauna.

Paleomagnetic samples were recovered from
several positions throughout the section at Leisey
(Figure 2). These were prepared according to the
methods cited earlier and were demagnetized and
measured in the Paleomagnetics Laboratory in the
Department of Geology at UF. In general, AF
demagnetization successfully isolated a
characteristic component of magnetization,
although in a few cases, subsequent thermal
demagnetization of the same sample was
required. Isothermal remanence acquisition
experiments indicated that magnetite was the
dominant magnetic mineral phase. All of the
paleomagnetic sites at the Leisey Shell Pit were
found to be of reversed polarity. This eliminates
the possibility that any of the sampled strata
represent the Olduvai subchron (normal polarity,
1.88 to 1.7 Ma). In combination with the other
chronologic evidence, it is probable that the
section at Leisey 3A and the bone bed at 1A lie
within the Matuyama Magnetochron.

Samples for strontium isotopic stratigraphy
were collected from eight positions at Leisey 1A
and seven at Leisey 3A (Figure 2). The Sr
isotopic ratio determinations ranged from 0.70905
to 0.70916. Clear trends toward higher ratios with
increasing height in the section characterize both
sites. This trend is consistent with published
curves for the late Neogene (e.g., DePaolo, 1986;
Capo and DePaolo, 1990; Hodell et al., 1991;
Figure 1). The lower shell beds at each site
contain molluscan faunas which, on the basis of
their "Sr/"Sr ratios, would seem to be of latest
Pliocene or Early Pleistocene age. The upper

FLORIDA GEOLOGICAL SURVEY

shell beds (above the bone beds) are clearly
younger, with ages ranging from Early Pleistocene
to early Late Pleistocene. Strontium isotopic
ratios from shells in the bone beds themselves are
compatible with the late early Irvingtonian Land
Mammal Age suggested by the mammals (Figure
1).

By integrating the geochronologic data from
these various and independent lines of evidence,
it is possible to constrain the age of the vertebrate
faunas at Leisey with a good degree of
confidence. The late Sappan assignment of the
mammalian component suggests an age of about
1.5 to 1.2 Ma. Placement in the Matuyama
Magnetochron (reversed magnetic interval above
the Olduvai subchron) further supports the
opinion that the fauna is no older than 1.66 Ma.
The Sr isotopic data confirm this age estimation
and strengthen the argument that the vertebrates
are not younger than about 1.2 Ma.

APAC SHELL PIT

One of the most spectacular fossil molluscan
faunas known to paleontologists is exposed in the
APAC Shell Pit (also referred to as, MacAsphalt
Pit, Newburn Pit, and Warren Brothers Pit) near
Sarasota, Florida. In Pliocene sands variously
referred to as the Pinecrest Beds (Olsson and
Petit, 1964; Petuch, 1982), the Pinecrest Sand
Member of the Tamiami Formation (Hunter, 1968;
DuBar, 1974), the Pinecrest Formation (Weisbord,
1972) or the Buckingham Formation (Petuch,
1987), occurs a rich, tropical fauna containing
over 200 species of bivalves (Stanley, 1986) and
over 600 species of gastropods (Petuch, 1987).
This site, located about 8 km east of Sarasota,
Sarasota County (E 1/2 sec. 12, T36S, R18E, Bee
Ridge Quadrangle), has been visited by
paleontologists for some 20 years and is widely
considered one of the most important
paleontological collection areas in Florida
(Petuch, 1987).

Because of its paleontological significance, it
is desirable to resolve the age of the APAC fauna
as precisely as possible. This has proven difficult
as diagnostic planktonic foraminifera and
calcareous nannoplankton are extremely rare or
not preserved in this marginal marine
paleoenvironment. Consequently, an integrative
stratigraphic approach was undertaken to address
the age question at this key site in a manner

similar to that previously described for the Leisey
Shell Pit. The results, summarized below, are
described in more detail in Jones et al. (1991).
From the preceding paragraph, it is clear that
fundamental problems surround the stratigraphic
identity and nomenclature of the various units at
the APAC Pit. I should emphasize that it was not
the intent of either this paper, or the research on
which it was based, to address these issues.

Invertebrate paleontologists have disagreed
on the age of the APAC faunas with estimates
ranging from Late Miocene (Olsson and Petit,
1964; Hunter, 1968) to Early Pliocene (Stanley,
1968) to middle or Late Pliocene (DuBar, 1974).
Petuch (1982) suggested the section may span
most of the Pliocene whereas Stanley (1986;
personal communication) believes it accumulated
fairly rapidly, during one transgressive phase
within the Early Pliocene involving a barrier island
- lagoon sequence. Stanley (1986) contends that
on the basis of index fossils the Pinecrest is
contemporaneous with the Jackson Bluff
Formation in northwestern Florida and the Duplin
Formation and the Yorktown Formation in the
Carolinas and Virginia. A similar suggestion was
made earlier by Hazel (1983) on the basis of
ostracode assemblages. A sparse calcareous
nannoplankton flora from the APAC Pit was
correlated by Akers (1974) to planktonic
foraminiferal zone N20, suggesting a middle
Pliocene age; however, the stratigraphic
provenance of the sample was not specified.
Similarly, Bender (1973) reported helium-uranium
dates on two corals from the "Pinecrest Formation
of Florida" (3.93 Ma and 3.49 Ma), but again no
locality data were provided. In fact, the corals for
the He-U dating probably did not come from the
APAC Pit (Druid Wilson, personal
communication).

The APAC Pit has also yielded a robust
vertebrate fauna which makes this site particularly
important in correlating marine and nonmarine
faunas. Over 5,000 fossil vertebrate specimens,
representing approximately 100 species, have
been collected from the pit, with most coming
from Unit 4 (Figure 3) or the "black layer" of
Petuch (1982). Bird bones account for about 40%
of the vertebrate species. Fish, reptiles, and
amphibians are also well represented.
Approximately 20 species of terrestrial mammals
offer the most useful biochronologic evidence.
Over half of the mammalian assemblage is

SPECIAL PUBUCATION NO. 36

Stylized stratigraphic section at the APAC Shell Pit from Jones et al. (1991) with brief
descriptions of the 11 units described by Petuch (1982).

characteristic of the Blancan Land Mammal Age
and all but four species are shared with two
previously known late Blancan sites in Florida
(Webb, 1974; Kurten and Anderson, 1980). Key
taxa include Nannippus peninsulatus, Trigonictis
macrodon, Platygonus bicalcaratus, and
Megalonyx leptostomus. The late Blancan age of
Unit 4 is further suggested by several species
representing typically Pleistocene genera,
including Geomys propinetis, Sigmodon medius,
Sylvilagus sp., Mylohyus floridanus, and Equus
(Asinus) sp., as well as characteristic taxa that did
not survive into the Irvingtonian, such as
Rhyncotherium and Nannippus.

Of further and considerable chronologic
importance are diagnostic, South American taxa
that crossed the Panama land bridge and appear
in the APAC Pit as immigrant taxa (Webb, 1985).
These Include the armadillos Holmesina
floridanus and Dasypus bellus, the ground sloths
Glossotherlum chapadmalense and Megalonyx
leptostomus, the capybara Neochoerus
dichroplax, and the raccoon Procyon sp.

Relatively precise dates for this suite of
immigrants are based on correlations with
stratigraphic sections at Mt. Blanco in Texas and
111 Ranch in Arizona which have excellent
radiometric and paleomagnetic control (Galusha
et al., 1984; Undsay et al., 1984). The first
appearance of this suite of neotropical Immigrants
coincides with the Gauss/Matuyama magnetic
polarity transition (Lindsay et al., 1987), forming
the base of the late Blancan at about 2.5 Ma. The
late Blancan ranges from 2.5 Ma to 1.9 Ma
(Lundelius et al., 1987; Figure 1), corresponding
with the lower (reversed) portion of the Matuyama
Chron. The vertebrate fauna from Unit 4 at APAC
falls within this interval.

A paleomagnetic investigation of the APAC
section was undertaken using the same
procedures employed at the Leisey Shell Pit. At
least three separately oriented samples were
collected from each of 13 sites throughout the
sequence. These were demagnetized and
measured on the cryogenic magnetometer at UF
according to the methods discussed earlier.

Eleven of the original 13 sites yielded samples
which could be analyzed. Results from all
analyzed sites indicate that the entire section
exposed at APAC is of reversed polarity. The
temporal significance of these results is discussed
below.

An additional component to the biochro-
nologic assessments at the APAC Pit (which was
not done at Leisey) was an investigation of the
ostracode fauna. A total of 3644 ostracodes (ca.
400/sample) were obtained from Units 2-10 for
biostratigraphic correlation with ostracode
assemblages from the late Neogene of the
Atlantic Coastal Plain (e.g., Hazel, 1983; Cronin et
al., 1984). Cronin (1990) recently summarized the
biostratigraphic ranges for 127 ostracode species
endemic to the eastern U.S. In order to use local
ostracode species for correlation to a standard
timescale, the lowest and highest stratigraphic
datums were calibrated to standard marine
planktonic foraminiferal and calcareous
nannofossil zones using planktonic microfaunas
obtained from the same coastal plain samples
from which the ostracodes were studied (Dowsett
and Cronin, 1990; Cronin et al., 1990). A total of
68 selected taxa from Units 2-10 at APAC were
compared to this compilation.

Units 10-5 contain 16 ostracode species which
constrain the age of these beds to younger than
3.5 Ma with a high degree of confidence. Species
from these units correlate with those from the
Duplin Formation of the Carolinas, the Raysor
Formation of Georgia, and the Jackson Bluff
Formation of Florida, suggesting that Units 10-5 at
APAC were most likely deposited around 3 0.5
Ma. There is a significant change in the
ostracode assemblages near the level of Unit 4
with an abundance of brackish water species in
Units 3 and 4, first appearances of several Late
Pliocene Pleistocene taxa in Units 4-2, and the
last appearances of many typical middle Pliocene
species. The assemblages in Units 4-2 are not
similar to those from the Duplin-Raysor-Pinecrest-
Jackson Bluff units and are most likely Late
Pliocene, younger than about 2.5 Ma.

Strontium isotopic analyses were run on
unaltered, aragonitic shell material from each of
Petuch's (1982) units. The ratios vary from
0.709089 at the top of the sequence to 0.709041
at the bottom. Two aspects of these data are of
significance in resolving the ages of the beds at

this site. First, the Sr ratios from Unit 1 clearly
indicate that it is younger than the rest of the
sequence, probably in the range of 1.0 2.0 Ma
(Jones et al., 1991). There may be a depositional
hiatus of several hundred thousand years between
Units 1 and 2. Second, the "Sr/"Sr ratios for
Units 3-11 are analytically inseparable.
Unfortunately the range of values
(0.709064-0.709041), including that of the bone
bed (0.709056), also overlaps the upper limits of
the "flat" portion of the global curve between
about 2.5 and 5.0 Ma (Figure 1). Strontium
isotopes alone, therefore, cannot resolve the age
of these beds. The pattern of Sr isotopic
variation, and the value determined for Unit 4 in
particular, lend confidence to the interpretation of
an age of ca. 1.9-2.5 Ma for the vertebrate-bearing
unit (Figure 1).

Taken together, all lines of evidence suggest
that the units at APAC record a complex
depositional history and that unconformities
throughout the section indicate some of the
record is missing. The Sr isotopic data,
ostracode assemblages, and the vertebrate fauna
of Unit 4, together with the reversed magnetic
signature, suggest that Units 4-2 belong within the
lower Matuyama Chron (ca. 2.0-2.5 Ma). Below
Unit 4, it is more difficult to constrain the ages.
Based upon the ostracodes, Units 10-5 appear
slightly older, perhaps 2.5 to no more than 3.5
Ma. The reversed magnetic polarity for each unit,
together with the biochronologic data, offers three
possibilities for the correlation of Units 10-5: 1) if
the faunas are around 2.5 Ma or younger, these
units could be assigned to the lower Matuyama
Chron, along with Units 4 and above; 2) if the
faunal age is actually around 3.0 Ma, a correlation
with the Kaena or Mammoth Subchrons within the
Gauss Chron is possible; or 3) if the faunal age is
closer to 3.5 Ma, these units could belong within
the upper portion of the Gilbert Chron. No
ostracodes were recovered from Unit 11 at the
bottom of the pit, but based on molluscan
evidence, this unit would appear to be older still
(Petuch, 1982; Stanley, 1986). At the other end of
the sequence, Sr isotopic data suggest that Unit
1 is appreciably younger than the beds below,
being deposited between about 2.0 and 1.0 Ma.
This age discrepancy was noted earlier by Petuch
(1982) who suggested the possibility of Unit 1
belonging to the Caloosahatchee Formation, an
opinion voiced more recently by Lyons (1991).

SPECIAL PUBLICATION NO. 36

CONCLUSIONS AND FUTURE DIRECTIONS

The richly fossiliferous strata of Florida's
Pliocene and Pleistocene record offer a unique
opportunity to investigate physical and biological
changes which occurred over the past few million
years of earth history, particularly in this region of
the world. The shallow marine deposits frequently
contain interbedded nonmarine units which
provide a chance to directly link marine and
continental records. If such correlations are to be
accurate, so that events in the terrestrial realm
can be calibrated with those in the marine realm,
adequate temporal control is required.
Unfortunately, this has proven elusive throughout
much of the late Neogene record in Florida
where, despite nearly a century of work,
Plio-Pleistocene stratigraphy remains mired in
controversy and confusion. Nomenclatural
problems abound, creating an almost intractable
situation.

To begin sorting out some of the temporal
difficulties in southern Florida, we elected to
pursue a multi-disciplinary approach, integrating
biostratigraphy with chemical and magnetic
stratigraphies. We chose to focus upon several
key sections which contained interbedded
terrestrial vertebrate and marine invertebrate
fossils, attempting to determine the age of these
important marine/nonmarine tie-ins as precisely
as possible. To date, two such sites have been
investigated in detail, the Leisey Shell Pit and the
APAC Shell Pit. By combining vertebrate and
invertebrate biochronologic assessments with
strontium isotopic stratigraphy and
magnetostratigraphy, it was usually possible to
constrain the age of at least part of the section at
each site to 0.5 m.y. Encouraged by these
results, we are currently expanding our studies to
Neogene strata likely to fall on segments of the
global strontium curve permitting higher temporal
resolution (i.e., rapid change in "Sr/"Sr), notably
the latest Pliocene and Pleistocene and parts of
the Early and Middle Miocene. Though certainly
not a panacea for the stratigraphic ills of the
Florida Neogene, we believe this integrative
approach has proven effective by marshalling
diverse and independent lines of evidence toward
resolving complex geochronologic uncertainties.

ACKNOWLEDGMENTS

Portions of this research were supported by
NSF Grants BSR-8314649, BSR-8711802, and
EAR-8708045 and were performed by the
individuals listed at the end of the Introduction
section. This paper represents University of
Florida Contribution to Paleobiology No. 389.

Jones, D. S., and Hasson, P. F., 1985, History and
development of the marine invertebrate faunas
separated by the Central American Isthmus: in,
Stehli, F.G., and Webb, S.D., eds., The great
American biotic interchange: New York, Plenum
Publishing Corp., p. 325-355.

Petuch, E. J., 1982, Notes on the molluscan
paleoecology of the Pinecrest Beds at Sarasota,
Florida with the description of Pvruella, a
stratigraphically important new genus
(Gastropoda: Melongenidae): Proceedings of the
Academy of Natural Sciences of Philadelphia, v.
134, p. 12-30.

A large number of sediment facies lie within
the poorly defined, Pliocene-age Tamiami
Formation of southwest Florida. There are at
least nine mappable members or facies ranging in
lithology from quartz sand to dolosilt to limestone.
Stratigraphically, the Buckingham Limestone
Member, a tan clay and quartz sand facies, and a
quartz sand facies can lie at the base of the
formation depending on the specific locations.
The Ochopee Limestone Member and a quartz
sand faces lie in the middle of the formation, and
a number of different lithologies containing the
Pinecrest fauna lie at the top of the formation. It
is probable that the isolated areas containing
well-preserved aragonitic shell are slightly younger
than the limestones containing only calcitic fossils
and molds and casts. The estimated age of the
Tamiami Formation ranges from 2.8 to 4.2 million
years.

INTRODUCTION

The Tamiami Formation is a poorly defined,
litho-stratigraphic unit containing a wide range of
mixed carbonate/siliciclastic lithologies and
associated fossil assemblages. A variety of
definitions have been given to the formation in
southwest Florida (Figure 1). The informal name
'Tamiami limestone" was first given to the unit as
observed in a series of sandy limestone outcrops
occurring along the north side of U.S. 41
(Tamiami Trail) in Collier County (Mansfield,
1939). Detailed historic descriptions of the
evolution of the Tamiami Formation definition are
given in Hunter and Wise (1980), Missimer (1984),
and Meeder (1987).

An objective of this paper is to question the
classical approach to the naming of members or
sediment facies for the purpose of correlation in
a relatively thin, 50 to 150 feet, mixed
carbonate/siliciclastic formation. The named
members of the formation are a series of biofacies
and lithologies that are mappable only over
limited areas. In some cases the thickness of a
given facies is less than 5 feet. In comparison to
formations of Miocene or older age in the Gulf
Coast, the entire Tamiami Formation is thinner
than most named members of any given
comparative formation. The naming of various
members, in general violation of the Code of
Stratigraphic Nomenclature, has lead to a major
state of confusion.

Controversy over definition of both the top
and the bottom of the unit, as well as the age,
have been a problem for the past 40 years. The
top of the Tamiami Formation can be any one of
a number of different lithologies, including
limestone, sandstone, quartz sand, mart, shell, or
clay. Most of the named members of the
Tamiami Formation, such as the Ochopee
Limestone and the Buckingham Limestone, are
devoid of preserved aragonitic fossils. Because
of the implied age difference between the
members of the formation devoid of aragonitic
shell and some differences in fossil assemblage,
Olsson (1964, 1968) placed the Pinecrest beds
stratigraphically above the Tamiami Formation.
Although there are several recognized
disconformities between various members of the
Tamiami Formation, the Pinecrest beds or
Pinecrest Sand Member should be contained
within the Tamiami Formation.

SPECIAL PUBLICATION NO. 36

SCALE
0 6 MILES
SCALE
0 10 KILOMETERS

Figure 1. Map of southwest Florida showing locations of sections.

COLLIER
COUNTY

FLORIDA GEOLOGICAL SURVEY

A major eustatic regression occurred world-
wide during the middle of the Pliocene or about
4.2 Ma (Vail, Mitchum, and Thompson, 1977) pro-
ducing a major disconformity throughout South
Florida. The disconformity separating younger
Pliocene and Pleistocene formations from the
underlying Hawthorn Group is distinctive
throughout most of southwest Florida, regardless
of the lithologies occurring below and above this
surface. The top of the formation is also bounded
by a disconformity.

The definition of the Hawthorn Group, as
described in Missimer and Banks (1982), Missimer
(1984), and Scott (1988), is utilized to establish
the base of the Tamiami Formation. In Sarasota
County, Charlotte County, and most of Lee
County, the base of the Tamiami Formation
occurs at the top of the first major green
dolosilt/sand unit, which occurs in the Peace
River Formation of the Hawthorn Group. In
southern Lee County, there is a green dolosilt unit
lying within the Tamiami Formation. In this area,
the top of the underlying Hawthorn Group occurs
in a gray quartz sand and clay unit. In Collier
County, the contact may be defined by the
occurrence of either a quartz sand unit or a green
dolosilt unit.

DESCRIPTION OF TAMIAMI FORMATION
FACIES

Introduction

At least 9 subsurface facies of the Tamiami
Formation have been mapped by various investi-
gators in southwest Florida to some extent on the
basis of predominant lithology. The stratigraphic
relationship of these units at a number of loca-
tions in southwest Florida is shown in Figure 2.
These units vary in terms of specific composition
and thickness, at a given location. Therefore, the
information presented in Figure 2 shows only the
vertical stratigraphic positions at some general
locations. Descriptions of some sections and well
logs used in this analysis are given in the
appendix (Table 1).

Pinecrest Sand Member

The name, Pinecrest Sand Member, is
derived from the "Pinecrest beds" informally
described by Olsson (1964, 1968) for a faunal

assemblage found near the 40-mile bend along
U.S. 41 near the Collier Dade County line. This
name for the faunal assemblage was pre-dated by
the description given by Tucker and Wilson (1932,
1933) for a similar faunal assemblage found near
the village of Acline in Charlotte County. The
lithology of the Acline and the original "Pinecrest
beds" was aragonitic shell and quartz sand with a
fine grain, green or gray-green, silt-sized matrix.

According to the Code of Stratigraphic
Nomenclature, members of a formation are to be
defined on the basis of lithology (American
Commission on Stratigraphic Nomenclature,
1972). Unfortunately, presence of the Pinecrest
fauna at any given location has been the basis of
defining a Pinecrest Member regardless of the
predominant lithology. Since the Pinecrest was
originally defined as the Pinecrest Sand, in this
paper, the lithology of the member is specifically
defined as a sand and shell unit. Deposits of
Pinecrest fauna, described in Sarasota County by
Petuch (1982) and Stanley (1986) are included in
this member, as well as the sand and shell unit
containing the "Acline fauna" in Charlotte County.
The reefal limestone faces containing Pinecrest
fauna and termed the Golden Gate Member of the
Tamiami Formation is treated as a separate
member.

Most members of a formation can be
mapped over some significant size geographic
area, perhaps at least several square miles. The
Pinecrest Sand Member, however, occurs in
rather isolated, small areas, commonly less than
one square mile in size, separated by large
distances. It is not continuous between areas
where the section has been measured and
described. For example, there are very isolated
occurrences of the Pinecrest Sand Member in
Charlotte County, along Alligator Alley in Collier
County, in Dade County, and in a few areas north
of the Big Cypress Indian Reservation in Hendry
County. The Pinecrest fauna found at Acline
occurred only in one shell pit, although at least
ten other nearby pits penetrating to near the top
of the Hawthorn Group showed no Pinecrest
fauna, only Caloosahatchee-age fauna. Test
borings made by the Florida Department of
Transportation across Alligator Alley also showed
discontinuous occurrences of the Pinecrest
lithology. At many of the isolated occurrences of
the Pinecrest Sand, it lies disconformably on
another underlying Tamiami Formation member.

SARASOTA

COUNTY

1. QUALITY
AGGREGATES
PIT

CALOOSAHATCHEE

PINECREST

SAND FACES

HAWTHORN

2. SHELL CREEK

CALOOSAHATCHEE

SAND FACIES

HAWTHORN

3. ACLINE

CALOOSAHATCHEE

ACLINE FAUNA

SAND FACIES

HAWTHORN

LEE COUNTY

4. NORTHWEST
AREA

5. ALVA AREA

FORT THOMPSON

BUCKINGHAM

HAWTHORN

6. LEHIGH ACRES

FORT THOMPSON
CALOOSAHATCHEE

UNNAMED LIMESTONE
(PINECREST FAUNA)

BUCKINGHAM
HAWTHORN

LEE COUNTY

COLLIER COUNTY

7. FORT MYERS
BEACH AREA

FORT THOMPSON

OYSTER FACIES
(HYOTISSA)

LIMESTONE

SAND AND CLAY
FACIES
HAWTHORN

8. SOUTH-CENTRAL 9. BONITA SPRINGS

FORT THOMPSON

UNNAMED UMESTONE
(PINECREST FACES)

OCHOPEE
BUCKINGHAMO
HAWTHORN

FORT THOMPSON

UNNAMED LIMESTONE
PINECREST FAUNA)

BONITA SPRINGS

OCHOPEE

HAWTHORN

10. MULE PEN
QUARRY

FORT THOMPSON

UNNAMED GOLDEN
LIMESTONE GATE

BONITA SPRINGS

OCHOPEE
HAWTHORN

11.ALLIGATOR ALLEY

FORT THOMPSON

PINECREST

UNNAMED LIMESTONE

BONITA SPRINGS

OCHOPEE

HAWTHORN

12.OCHOPEE

Figure 2. Tamiami Formation faces in southwest Florida.

CHARLOTTE COUNTY

FLORIDA GEOLOGICAL SURVEY

Unnamed Umestone Facies

A large area of southern and central Lee
County and northern Collier County is underlain
by a limestone unit that contains the Pinecrest
fauna. This limestone is light gray to tan in color,
contains 0 to 5 percent quartz sand and shells of
only calcitic fossils. All or most of the aragonitic
material has been dissolved, leaving mostly molds
and casts of the fauna and moldic porosity. The
limestone does contain a number of large oysters,
particularly Hyotissa haitensis (Sowerby, 1850),
barnacles, and pectens. The density of preserved
calcite fossils increases from the south to the
north part of the faces in Lee County.

This limestone unit is considered to be
stratigraphically equivalent to part of the Golden
Gate Reef Member as defined by Meeder (1987).
However, the preserved reefal material and
aragonitic fossil assemblage described by Meeder
do not occur uniformly in the limestone north of
the Bonita Springs area of south Lee County. The
corals are recrystallized, where present, and the
mollusks are in molds and casts. The
stratigraphic position of the Unnamed Limestone
Faces Is considered to be near or at the top of
the Tamiami Formation.

There is a distinctive disconformity between
this unit and the underlying Buckingham Lime-
stone Member in the Lehigh Acres area of north
Lee County. In south-central Lee County, the
limestone faces lies directly on top of the
Ochopee Umestone Member.

Golden Gate Reef Member

Meeder (1987) conducted a major geologic
investigation of the Tamiami Formation oriented
toward the mapping and faunal identification of
reefal assemblages found in Collier and Lee
Counties. Sixteen rock types were identified of
which fifteen occur within the Tamiami Formation.
The reefal material described by Meeder (1987)
does not occur uniformly over the area, and
varies from a well-preserved state in the Golden
Gate area of Collier County, where most of the
coral and associated molluscan fauna are
aragonitic, to a lesser state of preservation north
in Lee County, where the corals are calcitic and
recrystallized.

Meeder (1987) did not formally define the
Golden Gate Reef Member in terms of the
stratigraphy and the relationships with other major
lithic units. Therefore, the Golden Gate Reef
Member is considered to be laterally equivalent to
the Unnamed Limestone Facies with some
younger reefal sediments occurring at the very
top of the sequence. It differs in lithology from
the Pinecrest Sand Member and is a carbonate
rather than siliciclastic unit.

Bonita Springs Marl Member

The Bonita Springs Marl Member is a green
dolosilt unit in the vicinity of Bonita Springs, but
contains a large variety of different lithologies all
of which contain a lime mud or dolosilt matrix. It
was informally named by Missimer (1984). This
unit can be mapped over about a one hundred
square mile area in southwest Lee County and
northwest Collier County. In certain areas, the
lithology of this unit is similar to the uppermost
part of the Peace River Formation (Hawthorn
Group). It is devoid of microfossils, but does
contain barnacles which commonly occur in many
of the Tamiami Formation faces.

If this unit did not lie directly between the
Unnamed Limestone Facies and the Ochopee
Limestone Member, it probably would be mapped
on a lithologic basis as part of the Peace River
Formation which is the uppermost stratigraphic
unit in the Hawthorn Group (Scott, 1988).
However, the similarity in lithology is the probable
result of the incorporation of reworked Peace
River Formation dolosilt being deposited into a
shallow embayment occurring in the Bonita
Springs area. The lithology and color of the
"marl" in areas south of Bonita Springs closely
resembles modern carbonate depositional
environments such as Florida Bay.

Oyster (Hyotissa) Facies

Hyotissa haitensis is a common, large (15-25
cm height) fossil oyster occurring in most of the
Tamiami Formation faces. Hyotissa shells form
over 60% of the section in an area located in
western-central Lee County. This area covers 20
to 40 square miles and the thickness of the unit
reaches from 5 to 12 feet.

SPECIAL PUBUCATION NO. 36

The Hyotissa faces contains about 60 to
75% oyster shells, 5% Pecten shells, 5 to 20%
quartz sand and phosphate nodules, and 10 to
20% lime mud and clay. Only calcitic fossils
occur within the unit and there are few, if any,
molds and casts of aragonitic fossils. The matrix
of the unit is a lime mud similar to the
Buckingham Limestone Member.

Ochopee Limestone Member

The Ochopee Limestone Member is a major
faces of the Tamiami Formation and can be
mapped beneath most of Collier County and parts
of Lee, Hendry, Dade, Monroe, and Broward
Counties (Hunter, 1968). It occurs at land surface
in Collier County along the Tamiami Trail (U.S.
41), where Mansfield (1939) first described the
Tamiami Formation. It was originally described as
a "light gray to white, hard, sandy limestone (a
calcarenite) containing abundant identifiable
mollusk molds and well preserved pectens,
oysters, barnacles, and echinoids".

A distinguishing characteristic of the
Ochopee Limestone Member is the occurrence of
fine to very fine quartz sand within the unit. The
percentage of sand in the member can range
from 5 to 80% depending upon the location and
depth. Commonly, the percentage of quartz sand
increases with depth.

Sand Facies

A large area of coastal Charlotte and Lee
Counties is underlain by the Sand Facies of the
Tamiami Formation. The lithology at a given
location can vary greatly, but the common
characteristic is a matrix of medium to fine quartz
sand. This lithology was described by DuBar
(1962) and illustrated in Hunter (1968).

The Sand Facies of the Tamiami Formation
was directly observed in several dewatered pits
located along Burnt Store Road in Lee and
Charlotte counties and in various locations within
Cape Coral in Lee County. There are no
preserved aragonitic fossils found in this unit. The
most common calcitic fossils found are barnacles,
oysters, (Hyotissa haitensis, Ostrea disparilis),
pectens (Argopecten eboreus, different varieties),
bryozoans, and the gastropod Ecphora. Bedded

sandstone and very thin limestone units are
common with this section. At a number of
locations the Sand Facies is a partially lithified
barnacle shell hash and sand unit.

Buckingham Limestone Member

The Buckingham Limestone Member was
originally described by Mansfield (1939), and was
informally defined as a member of the Tamiami
Formation by Hunter (1968). The description
given by Hunter (1968) is "Light gray to white,
soft, calcareous clay (a calcilutite) that weathers
to a buff colour. It contains some quartz sand, a
few bone fragments and shark teeth, and few
grains of brown phosphate. Poorly preserved
fossil molds, and well preserved pectens, oysters,
barnacles, echinoids, etc. are present". A series
of cores taken through the unit in the vicinity of
the W.P. Franklin Dam in Lee County show that
the unit also contains a large quantity of rock
fragments (predominantly phosphatized
limestone), quartz sand, phosphatic pebbles, and
reworked green dolosilt.

A large quantity of the sediment contained
within the Buckingham Limestone Member is
reworked from the underlying Peace River
Formation. It contains a matrix of lime mud and
clay minerals, but over 70% of the sediment is
quartz sand or rock fragments at many locations.
The member has a very high concentration of
radioactive materials (phosphate nodules) and
can be easily distinguished from the underlying
Peace River Formation (Hawthorn Group) dolosilt
using gamma ray logs.

Tan Clay and Sand Facies

At a number of locations in central, coastal
Lee County, a tan clay and sand faces underlies
either the Oyster (Hyotissa) Facies or the
Unnamed Limestone Facies. This faces lies
disconformably beneath the overlying faces with
a very thin calcrete at the top of the unit. The
quartz sand within the unit contains a number of
concretions, some containing rior sparry
calcite crystals. Grain size of ine quartz sand
ranges from very fine to coarse. The phosphatic
material contained within the unit is also
sand-sized.

FLORIDA GEOLOGICAL SURVEY

STRATIGRAPHIC CORRELATION OF
DEFINED MEMBERS

There are a large number of different
sediment faces that lie within the Tamiami
Formation in southwest Florida. The relative
stratigraphic positions of the described members
at various locations have been shown in Figure 2.
Of the nine different stratigraphic units described,
only 1 to 4 occur in a vertical stratigraphic section
at any given locality. Therefore, it is difficult to
correlate the units on a relative age basis. Fortu-
nately, there are some observed disconformities
lying between some of the members and fossil
correlations that assist in correlating the
lithologies.

Based on the observed stratigraphic posi-
tions, the location of disconformities, the pre-
servation of fossil material, and the various
lithologies, the implied stratigraphic relationships
of the members have been determined (Figure 3).
A plan view of the occurrence of the uppermost
Tamiami Formation unit encountered in Lee
County is given in Figure 4. The Buckingham
Limestone is the lowermost Tamiami Formation
member based on the character of the sediment
and the relative stratigraphic position of the unit at
various locations. It is the probable age
equivalent to the Tan Clay and Sand Facies and
perhaps to all or part of the Sand Facies. The
Oyster (Hyotissa) Facies lies as an equivalent of
the upper part of the Buckingham or immediately
above it. The Ochopee Limestone lies between
the units containing the Pinecrest fauna and the
underlying Buckingham Limestone. The upper
part of the Sand Facies is probably equivalent to
the Ochopee Limestone. Where the Unnamed
Limestone (Pinecrest fauna) lies directly on
Buckingham Limestone, there is a distinct
disconformity. All of the upper units containing
the Pinecrest fauna are quite close in age, but the
units containing well-preserved aragonitic shell
and coral are probably slightly younger than the
other units.

The lithology of the uppermost occurrence
of the Tamiami Formation in Lee County shows
some interesting relationships, although areas
delineated on Figure 4 are only approximate, and
there are some deviations within the areas shown.
A large part of the Caloosahatchee River valley
does not contain any Tamiami Formation sedi-

ments. The oldest Tamiami Formation unit, the
Buckingham Limestone Member, lies near the sur-
face only in the northeast part of the county. The
more siliciclastic facies lie mostly adjacent to the
existing coastal area, probably because of the
higher energy in these areas. Occurrences of the
Pinecrest Sand or Golden Gate Reef Members are
limited to isolated areas (these are the areas with
preserved aragonitic shell) where wave energy
and circulation are lower or the siliciclastic
sediment supply was limited. Some of the limited
occurrences of the Pinecrest Member may be
tempescles.

AGE OF THE TAMIAMI FORMATION

Most researchers currently consider the
Tamiami Formation to be Pliocene in age
(Missimer, 1978; Petuch, 1982; Meeder, 1987;
Ketcher, 1990). A lower part of the
Caloosahatchee Formation has been found to
have an age of about 2.2 Ma based on a Blancian
vertebrate assemblage found in Lee County (T.
Missimer and G. Morgan, personnel
communication). Based on this age data and the
sea level curve suggested by Vail, Mitchum, and
Thompson (1977), the top of the Tamiami
Formation probably occurs at about 2.8 Ma. The
lowermost age of the Tamiami Formation is
probably about 4.2 Ma based on the global sea
level curve of Vail, Mitchum, and Thompson
(1977) and some preliminary diatom age dates on
the upper part of the Hawthorn Group in South
Florida (Klinzing, 1987). There is considerable
uncertainty with regard to this age range.
Additional data must be conducted on age
diagnostic fossils and perhaps paleomagnetic
studies in order to better resolve the age of the
Tamiami Formation.

DISCUSSION ON STRATIGRAPHIC
CORRELATION OF PLIOCENE
AND YOUNGER FORMATIONS IN
SOUTH FLORIDA

From the mid-Pliocene (4.2 Ma) to present,
a complex series of sediment sequences have
been deposited on the South Florida platform.
The deposits are mixed carbonate and siliciclastic
sediments, representing a wide variety of shallow
water environments, including marine, brackish,
and freshwater. There are numerous discon-
formities, both regional and local, that separate
the sediment sequences. Many different sediment

faces have been given formal or informal member
names based on a conventional stratigraphic
treatment. A major problem with the current
framework of these sediments is that the total
thickness of the section is to a large extent less
than 50 feet thick in southwest Florida. Within
this 50 foot thick section, there are three or four
named formations, including the Pamlico Sand,
the Fort Thompson Formation, the Caloosa-
hatchee Formation, and the Tamiami Formation.
Each formation is further subdivided into a series
of members, some of which are only 1 to 2 feet
thick. This very confusing situation is further
exasperated by the mixing of lithostratigrahpic
and biostratigraphic definitions of members.

It is time to reassess the stratigraphic
methodology applied to the thin section of mixed
carbonate-siliciclastic sediments in southwest
Florida. Lithostratigraphic units must be defined
based on separation by disconformities and a set
of specifically defined lithologies. The approach
to description of the sediments in the future
should parallel the methods used in deep sea
stratigraphy with a lithostratigraphic framework
containing time lines established by fossils,
disconformities, and age dates.

ACKNOWLEDGEMENTS

Warren Allmon, Thomas Scott, Donald
McNeill, and Robert N. Ginsburg reviewed the
manuscript. I wish to thank them for their
comments, criticisms, and suggestions. I
especially wish to thank the late F. Stearns
MacNeil for the time he spent in the field with me
during his later years. His insight into the
Pliocene stratigraphy of southwest Florida was an
important influence on this compilation.

Petuch, E. J., 1982, Notes on the molluscan
paleoecology of the Pinecrest beds at Sarasota,
Florida with the description of Pyruella, a
stratigraphically important new genus
(Gastropoda: Melongenidae): Proceedings of the
Academy of Natural Sciences of Philadelphia, v.
134, p. 12-30.

The Plio-Plelstocene sediments of west-
central Florida are contained In a single
hydrostraligraphic unit, the surficial aquifer
system. Although the underlying Fioridan aquifer
system is the rmjor aquifer in terms of water
supply, the surficlal aquifer system is important in
a number of ways. First, the surflcial aquifer
system supplies many small wells for such
purposes as domestic uses, lawn Irrigation and
livestock watering. Second, the surficial aquifer
represents one of two routes of recharge to the
Floridan aquifer; the other route is through
sinkholes. Third, the surficial aquifer system,
because it lies close to the ground surface, is
easily contaminated.

Quantitative consderatlons of ground-water
How in any aquifer require knowledge of bs
hydraulic properties. Chief among these is
hydraulic conductity (K). the constant of
proportionality In Darcy's Law. How variable is K
for the surficlal aquifer system? Hydrogeologlits
who model multi-layer ground-water fows using
cells that represent I-square mile areas; can they
use the same value of K for the surficlal aquifer in
one county as in another? The number that
represents a cell of a regional model: how does
that relate to the range of values seen in a single
site within the area represented by the cell?
These kinds of questions require an evaluation of
the Ilthologic variation and Its effect on the
variability, or heterogeneity, of K.

Normally, questions concerning lithologic
variation can be approached by turning to the
stratigraphlc literature. In the case of west-central
Florida, the exercise Is frustrating. There are
many stratgraphlc names and considerable
discussion -- but few geologic descriptions.
Moreover, as pointed out by Missiner (1964) and
Scott (1990. this volume), the nomenclature and
discussions hinge on fossils, not on lithology.

In order to assess lithologlc varlablity as it
may affect K of the surficial aquifer system, we
have abandoned the literature and have begun an
ad hoc study using available wel logs and site
descriptions. The first results are presented here.
The study area Is the Southwest Florida Water
Management District (SWFWMD). We address
two scales of heterogeneity: (1) regional
variations (.e., SWFWMD-scale) in lithofacles as
revealed In vertically integrated, lithofacies maps
of the type developed by Krunmben and Sloss
(1963); (2) local variations (i.e., sie-scale) due to
sediment-texture properties within what on a
regional scale would be considered a homoge-
neous faces,

SURFCIAL AQUIFER SYSTEM

The Southeastern Geological Soclety's ad
hoc Committee on Florida Hydrostratigraphlc Unit
Definition (Southeastern Geological Society
[SEGS), 19W6) has defined the three hydro-
stratigraphic units making up the Florida section.
The surficial aquifer system is defined (SEGS,